Simple Model of How the Sky Changes Color at Sunset

The Sunset Egg is a fun and engaging demonstration on the science of light. The egg is made of “opalescent” glass. This refers to the way it looks different at different viewing angles (similar to the gem’s properties).

egg1

An oil-polished Sunset Egg, lit from below, shows both blue sky and yellow sun.

To use the egg hold it in one hand and close your hand around it. The egg will appear blue. Now hold it up to a source of white light, such as overhead lights. The egg will appear yellow. What’s going on? How can this be used to explain the blue sky and the yellow sunset?

The Sunset Egg responds differently to light based on its wavelength (The sky does the same thing). This process is called Rayleigh Scattering and when light is scattered, the shorter wavelengths are scattered more often.

When light hits the egg, more blue light is scattered than red and yellow, thus the egg usually looks blue. But the light that passes through the egg has had its blue light scattered away. The remaining light is yellow and red. Looking through the long end of the egg or using multiple eggs can also increase the effect.

sun earth diagram

The yellow sunset and the daytime blue sky are caused by different path lengths through the atmosphere. The longer the path, the less blue light remains.

But how can the egg help explain the sky?

During the day the light we see in the sky comes from light being scattered by air molecules (mostly oxygen and nitrogen). Since shorter wavelengths get scattered more often, the blue light is more frequently scattered. During sunset, the light has to pass through a more of the sky and that journey causes the blue light to get scattered out sideways on its way. The result is yellow and orange sunsets.

sunset

Sunlight that has lost its blue looks yellow.

This is easily seen in the egg. The light scattered sideways is blue, but the light traveling all the way through is yellow. In the case of the sky, the light is being scattered on air molecules, mostly oxygen and nitrogen, but also dust and other particulates. In the case of the egg, the light is being scattered on fine dye particles inside of the glass.

egg in light

The blue sky effect clearly shown on the top half of the egg. Note that the scattered blue light (moving to your eye) is perpendicular to the incoming light. The light that passes all the way through the egg is very yellow.

The egg behaves like a little piece of the sky, and it looks like one for the correct reason – scattering. When the light passes through a small bit of it, the egg or sky looks blue, but when light passes through a lot of it, whether it is the egg or the sky, it looks yellow.

aquarium

A similar blue sky effect can be achieved by using an aquarium full of water with a little coffee creamer. When light passes through the aquarium it gets scattered by the tiny coffee creamer particles. But blue light gets scattered more frequently, making the aquarium look blue over all.

Sunset effect

The light that passes through the aquarium has less blue in it and so it looks yellow. This causes a sunset effect. It is not just an effect however; this is the real cause.

When you first get the egg, it can be used immediately for these experiments. However, it might have a sheen of white dust. This can be washed off somewhat, but it is helpful to wipe cooking oil over it and then dry it off with a paper towel. This will give the egg a smooth surface and improve the demonstrations that follow.

Egg polishing

Cooking oil provides polish for a dull egg.

The reason the cooking oil smooths out the opalescent glass egg is because oil and glass have nearly the same index of refraction; they bend light by the same amount.

The Sunset Egg may be the best science gift ever because it is so much fun and can teach us so much.

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Newton’s Eye Poke Experiment [W/Video]

eye_poke_600

Repeat a classic experiment from the master himself, Isaac Newton. In 1666 Newton first performed the Eye Poke experiment, writing about it much later in his book OPTICKS. He used this experiment to support his theory that light was composed of particles. But today we will use it to demonstrate that our eye is indeed seeing INVERTED IMAGES.

Newton’s Eye Poke Experiment proves how we see upside down due to the nature of our eye’s convex lens.

This video was performed by Anna Spitz, and was written and directed by James Lincoln, as part of the AAPT Films series.


James Lincoln

Tarbut V’ Torah High School
Irvine, CA, USA

James Lincoln teaches Physics in Southern California and has won several science video contests and worked on various projects in the past few years.  James has consulted on TV’s “The Big Bang Theory” and WebTV’s “This vs. That”  and  the UCLA Physics Video Project.


 

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Behind the Scenes with Light & Color: 10 Great Demos

Introduction

Light and Spectrum is a common topic among all of the sciences. You will find a chapter devoted to it in Astronomy, Chemistry, Physics, and Biology. Therefore, enhancing your ability to teach this topic is going to benefit every member of the science department.

James

The RSpec-Explorer empowers teachers to have their entire class to experience quantitative spectroscopy at the same time and in a meaningful way. Up until now, it was very difficult to manage to get more than one person to be sure they were seeing the same thing through a diffraction grating or a refraction table. But with the RSpec-Explorer you can easily point out features in a gas tube line spectrum, a sodium lamp, or anything you can think of.

a lemon, an apple, and a green pepper are being studied using the RSpec-Explorer

In this picture, a lemon, an apple, and a green pepper are being studied using the RSpec-Explorer. The yellow lines in the left picture indicated that the apple is currently being investigated. The graph on the left shows that the apple is lined up (on the yellow line) and that its spectrum is mostly red, with a peak around 625nm.

In this article, I provide 10 examples of experiments you can do on light and spectrum, all of which are made easier by using the RSpec-Explorer.

1. Experiments on Color

white light of the iPhone flashlight turns out to be deficient in the light blues

The white light of the iPhone flashlight turns out to be deficient in the light blues. Unlike the even spread of color that would come from sunlight.

One of the first experiments you should do is to demonstrate that white light is made of colors. The term “white” is often used by scientists to refer to a light source that emits or reflects all visible wavelengths (400-700nm). However, the human eye cannot distinguish this real white light from a light source that is made of only a few colors. For example, if you examine a cell phone flashlight feature through a diffraction grating (such as the one on the RSpec Explorer’s camera) it will reveal that this apparently “white” light is actually missing some of the deep blues. Also, if you look at a “white” fluorescent lamp tube, it will reveal that it is made of several distinct colors but not a broad spectrum (like say for example a sunbeam, or a white incandescent lamp).

If you have a color mixing device (three colored lamps would work, or three lamps with filters) you can demonstrate to yourself that “white” light can be created by mixing Red, Green, and Blue (RGB). This is how a cell phone screen makes white light, and a computer screen, and most projectors! There are many sources available for this experiment.

Magenta

Magenta being faked by mixing red and blue. The pinkish looking light is diffracted revealing it is a mixture of red and blue. The spectrum on the right reveals the peaks of these two colors: 425 and 610nm.

A better trick is to mix just two colors and get a new color that will completely fool the eye. A major example is to mix red and green light and make “yellow.” I put the quotes here again because it only appears yellow – there is nothing yellow about green and red light mixed, except that it can fool the eye by appearing to be yellow. Mixing red and blue light makes “magenta” light and mixing blue and green makes “cyan” (again the quotes describe the appearance not the reality of the light).

The advantage of analysis through a diffraction grating is that it can easily discern the two colors, which diffract differently. The spread or “dispersion” of the light is linearly-dependent on its wavelength (to a good approximation). That’s how we can separate the light by its wavelength and reveal whether we are looking at a true color or only a synthesized one.

2. Ionized Gases

Ionizing Gases to display their spectra is an important activity in most science classes. Of course, you want to point out that different gases have different spectra and these can be used for identification. Every noble gas was identified first based on its spectrum. (How else could you tell Neon from Argon, seeing as how they are both chemically inert?!!)

Gas tubes discharging

Gas tubes discharging: Hydrogen, Helium, Neon, Mercury.

The most important example is hydrogen, which is not a noble gas, but which has a readily recognizable spectrum. The Balmer Series (n=2) is the visible portion of the spectrum. It has a very obvious and bright cyan (486nm) colored line, a somewhat less bright red line (656nm), and a few violet lines (434nm, 410nm). Invisible is the Lyman (ultraviolet, n=1) and the Paschen (infrared, n=3) series. The hydrogen spectrum is important, not just because it is so familiar, but because it can be calculated easily (using the Rydberg formula), and it was also the spectrum that was used by Niels Bohr when he applied quantum theory to explain atomic spectra for the first time.

The Balmer series

The Balmer series for hydrogen contains the four visible lines of hydrogen’s spectrum and all of these transitions involve the n=2 orbital (marked in yellow).

The Rydberg Formula

The Rydberg Formula for Hydrogen’s spectral wavelengths

Helium, on the other hand, is not as familiar but can be made so by learning to recognize it by its bright yellow (589nm) line. Also, the story that the helium absorption lines were first seen in a solar eclipse is a good history of science tidbit. That is how helium got its name, from the sun god – Helios. Also, helium looks yellow-pink when ionized, where hydrogen usually looks red-purple.

four characteristic spectral lines

The four characteristic spectral lines in the Balmer series for Hydrogen.

Recognizing Helium

Learning to recognize Helium based on its very bright yellow discharge line and its pale yellow spectral tube.

Neon is amazing to look at even without a diffraction grating. Its pastel-electric red glow earned it its name as the “new” element for the electric age. The diffraction grating reveals that it is saturated with reds, yellows, and a few scattered greens.

In the plasma globe you can find ionized gases and with the RSpec-Explorer and (if you can line it up carefully) you can identify the gases inside (helium is the main one).

paper clip bent

A paper clip bent to include a small loop does a great job of carrying salt to the flame.

It is also possible to burn salts and reveal the spectral lines.  The most obvious salt is table salt, sodium chloride, which burns well in a paper clip loop held over a candle flame. Teachers often dissolve a lot of salt in a little water which can sometimes help (dissolved ions have more surface area/volume than crystals so they burn easier). The yellow sodium “doublet” (two very close wavelengths at once) easily identifies it. You can also recognize sodium in yellow streetlights at night. Other salts that emit good colors will contain copper, strontium, calcium, potassium, and iron. Which you can usually find in the chemistry storeroom. All of these are often used in fireworks (usually mixed with magnesium and gunpowder) and if you need help getting the fire hot enough, you should try dissolving them in methanol. Safety first! Be sure to have safe water on deck for emergencies and a fire extinguisher is a good idea, too.

3. Investigate Different Light Bulbs

These days people are very interested in how all the different types of light bulbs make light. Diffraction is the best way to identify how the light is made. If you look at an ordinary incandescent bulb you will see it has a broad spectrum with a lot of yellows and reds giving it a “warm” glow. On the other hand, fluorescence light bulbs contain mercury and will have several easily recognizable spectral lines that correspond to that element. Mercury is a good choice for fluorescence because the many energetic purples and UVs in the spectrum can give energy to fluorescent paints which reradiate that energy as visible light. If anyone doubts that there is mercury in our light sources, they should be easily convinced by this demonstration!

mercury discharge tube

A mercury discharge tube demonstrates several violets and greens. But very little red and yellow. Invisible is the UV.

 

fluorescent light bulb

A fluorescent light bulb shows many of the same spectral lines revealing that it contains mercury.

Compared to incandescent bulbs, fluorescent bulbs tend to make people look drained of color. This is because the high amount of blues and purples can cast an “unhealthy” purple glow on you. A 100W incandescent bulb nearly imitates the sun’s spectrum. Which peaks in yellow-green, giving you that healthy glow.

Incandescent Bulb

An incandescent bulb reveals its warm colors by peaking in the reds and yellows.

You can also investigate other light sources such as white diodes (which have a lot of purples because they fluoresce, too), yellow sodium parking lot lights, or even a plant light. Plant lights aim to provide the two spectral colors of photosynthesis – blue and red. Green plants reflect green light and thus they do not absorb it for making glucose. Red light also provides a signal to the plant to let it know that the day is long enough (i.e. spring or summer) to start investing itself in growth. (Some plants actually suppress their growth in summer to take advantage of a less competitive winter season.) Anyways, plant lights provide these non-green colors in high supply.

4. Analyze the Wavelengths of Lasers and Diodes

Light Emitting Diodes are a ubiquitous source of light in our lives. In most cases, diodes will be sold to emit a specific wavelength of light but in actuality, there will be a spread of color about this “nominal” value. (Nominal is an engineering term meaning “named” or expected, as opposed to what actually results during the experiment.) For example, a “626nm” LED might emit 96% of its light between 610 and 632nm. This amount of spread can be measured by the RSpec-Explorer and it’s interesting to compare this with laser light.

orange diode

The orange diode demonstrates that its wavelength is quite spread out over the 30nm that surrounds its nominal value.

A HeNe laser

A HeNe laser demonstrates both that it is monochromatic and that it contains neon by emitting the 626nm red that helium lacks.

Lasers have “monochromatic” light. This means that it is very nearly only one specific wavelength. These wavelengths are usually listed on the laser itself. A good experiment would be to verify that the wavelength printed on the laser is actually the wavelength it emits.   Even diode lasers are usually quite monochromatic. “Lasing” requires the light to be nearly one wavelength – lasers are a good example of light standing waves.

helium neon laser

A bare helium-neon laser glows yellow pink with helium but emits a neon red beam.

When it comes to red lasers there are many different types. Helium-Neon lasers will have different wavelengths than red diode lasers. You can use the RSpec-Explorer to prove that it is actually neon that emits the red light in the helium-neon laser. This is a good demonstration of the power of spectral analysis to identify elements. If you have a bare helium-neon laser it can be particularly engaging in this activity.

5. Investigate Fluorescence

tonic water

A violet laser energizes the quinine in tonic water.

Fluorescence is always an engaging activity.   Energetic light (such as UV or violet) lands on a substance that can absorb it and that energy is re-emitted as less energetic visible light. For example, a black (UV) light might shine on your socks (which have fluorescent detergent) in them and then white visible light will be emitted.

Good candidates for investigation with the RSpec-Explorer include tonic water, highlighters, extra virgin olive oil, Willemite, and phosphorescent vinyl sheets. All of these will glow under UV or violet light (such as a violet laser or black light) but the olive oil works better with a green laser (the yellow olive oil absorbs violet light very quickly).

light off

Above: UV Light is turned off. 

Lights On

A few fluorescent rocks with UV turned on. Willemite is in the middle.

Phosphorescence is a special type of fluorescence in which the emission of the light is suppressed for an extended period of time (the atomic transition is slower). In fluorescence, the emission of light is nearly instantaneous. In either case, the wavelength of the emitted light is always longer – less energetic. The words phosphorescence and fluorescence are only historical. Not all phosphorescent materials contain phosphates (though most do) and not all fluorescent materials contain fluorides (though many do, including toothpaste). Willemite, which is a fantastic glow rock, contains neither phosphorus nor fluorine.

6. Measure Temperature Using the Blackbody Curve

Turn on your electric oven, toaster, or electric stove and it will first glow red-hot, then yellow-hot, and if we went further it would glow white-hot. This change in color with temperature was described mathematically by Lord Rayleigh, James Jeans, Wilhelm Wein, and finally Max Planck. The Rayleigh-Jeans Law described the long wavelengths and Wein’s Law described the short wavelengths, but both “laws” failed outside of those conditions. Planck was the first to solve the emission problem for ends of the curve. Planck’s function is also called the “blackbody” curve because even a black object will be seen to emit light in these proportions if it gets hot enough.

We can use this curve to determine how hot our light bulbs are. The problem is that most of the light they emit is infrared which is generally not visible. If you are willing to accept that there is an enormous amount of invisible light, then you will be able to approximate the temperature of a glowing hot object by this method. You may be surprised to find out that even little circuit-lab style bulbs are actually heating up to about 4000K – but this is consistent with theory.

Reference Library

The Reference Library includes Planck Curves which can be used to fit to our spectrum. Hotter objects have steeper slopes on the left side. The right side is incomplete due to invisible infrared light.

I am not saying that you can get a highly accurate measurement with the RSpec-Explorer camera, but you can approximate the temperature reading, and probably within 15% (be sure to turn the brightness down in the settings). It is impressive that lightbulbs get this hot to glow. It also helps us appreciate why they must be contained in bulbs – if they were exposed to the oxygen in the air at these temperatures they would immediately burn up and break the circuit.

It’s fun to compare these light bulb filaments to the temperature of the sun which is a G2V star (there is a star reference collection in the References, too). The temperature of the sun can be determined from the black body curve as well. In fact, this is how we measure the temperature of the sun – at least on its surface!

Measuring temperature using the blackbody curve is a good way to get Modern Physics concepts into your classroom.

7. Diffraction Experiments

In Astronomy and Physics, the idea of Diffraction is a commonly taught subject. Diffraction of light is one means by which we can separate it based on its wavelength. A diffraction grating is made when a laser cuts tiny grooves into the surface of a piece of plastic or glass. A good example of one is a CD.

Some themes of diffraction are that the smaller the distance d between the grooves, the more dispersion, X, you will get. The light will spread apart further from its straight line path. Also, the longer the wavelength, λ, of the light the more easily you can disperse it. And of course, the more space it has to travel before it lands on a screen the more it will disperse. This length is usually called L (the distance to the screen or camera). All of these ideas come together in the diffraction formula:

 

Xm = m λ L / d

 

where m is an integer, usually 1, that tells you the “order number.” We need m because the pattern will repeat itself about twice as far out, and that is called the 2nd order. Usually, the 2nd order is much less bright than 1st order diffraction. This formula is an approximation, assuming that the light is not being diffracted at large angles from the straight-ahead path, it works well for angles under 30 degrees (ie first order).

measure both

You’ll have to measure both the dispersion X (left) and the distance to the camera L (right) if you wish to apply the diffraction formula. The wavelength λ is given in this case, which is unusually convenient. What is not given is the groove spacing d.

A good lab would be to use this formula to try to measure the line spacing “d” for the diffraction grating of the RSpec-Explorer camera. The units should be in meters/groove or meters/line. Use meters as the unit for X, λ, and L.

 

8. Measure the Wavelength of Infrared

The wavelength of Invisible Infrared Light can be measured with a diffraction grating and a digital camera (which can see the infrared light). But, the RSpec-Explorer makes this easier because it can tell you the wavelength based on the distance the light is diffracted on the video screen. A TV remote control can provide a source of near-infrared light (should be between 800 and 1000nm) but I have had more success with loose infrared diodes because I can crank up the voltage and get them to shine very bright. To ensure success, have a very dark room with the diode close to the camera. Also, when you rotate the camera you should be able to see the first order diffraction of the infrared light.

infrared diode

An infrared diode (bright circle) and its lens flare (dimmer circles) are plainly visible in this screen capture. Lens flares occur when bright lights are improperly focused by lenses. Like all cameras, the RSpec-Explorer demonstrates this feature.

To perform this experiment, get your diode showing up very bright, and line it up on the yellow calibration line. You will not be able to see the diffracted light because it will be diffracted so far that it will not appear on the screen. So instead we will have to recalibrate the camera to see further than the visible spectrum. First, line up a second light source with a familiar visible spectrum such as a Hydrogen gas tube or a white diode. Then, once you have it on the yellow line, rotate the camera to the right. This should cause the two light sources to be off-camera to the left but the color spectrum will remain. Based on your knowledge of that familiar spectrum you can recalibrate the camera. Go to Tools à Calibrate à Linear. Click on the first familiar pixel and enter its wavelength (in the case of hydrogen this would be the 486nm cyan line). Then click on the second pixel and set that to another wavelength (in the case of hydrogen this would be the 656nm red line). Now “apply” that calibration and close the window. Turn off your familiar light source and the infrared light should now be visible around 900nm. Be sure to move the yellow bars to sample it and explore its peak brightness. Like all diodes, infrared ones have a significant spread.

explore the infrared

A screen capture took after recalibrating the camera to explore the infrared. In this case, the hydrogen spectrum was used as a familiar reference. The next step would be to double check that the infrared diode was lined up with the hydrogen tube at the origin, and then turn off the hydrogen tube to reduce accidental light contamination.

 

9. Experiments that use the Intensity Feature 

Crossing Polarizers

Crossing Polarizers can reduce the Intensity of the light that comes through.

You probably have already noticed that brighter spectral lines show up as higher peaks on the y-axis. This is particularly obvious with the hydrogen spectrum’s cyan line at 486nm which is much brighter than its red 656nm line. This intensity reading feature can be helpful in other experiments as well. To take advantage of this feature, be sure to turn off the “Auto-Scale Y-Axis” feature on the bottom right panel of the screen.

A good experiment to try out is to block a light source with two polarizing sheets. When the polarizers are rotated they block more of the incoming light. As the relative angle increases (to 90 degrees) the blocked light source dims to nearly zero transmittance. This reduction in brightness is supposedly dependent on the square of the cosine of the relative angle between the polarizers. This intensity function I(θ) = Imax cos2(θ) is known as Malus’ Law. It is helpful if the source of light is monochromatic.

Another experiment that you can try (one that probably belongs in a Chemistry class) is that a higher concentration (molarity) of dye will block a proportionately larger amount of light. This is known as Beer’s Law. It might be best to start with a clear water sample for calibration then slowly add dye. I have had a lot of success with Coca-Cola. Again, it is helpful if the source of light is monochromatic.

Beer’s Law Experiment

In this Beer’s Law Experiment, the concentration of the solution is increased, causing the intensity of light to be decreased. Moussing over the central plateau makes an intensity measurement. It is important to set a constant scale for the y-axis. I have chosen not to fill the graph with color because the wavelength of light is not being measured, only brightness.

 

10. Astronomy Experiments

The RSpec software that is employed with the Explorer camera was originally produced to serve astronomers. Thus, there are many vestigial traces of this in the reference libraries and in the training videos that accompany the device. It can be fun to take advantage of these features and see how far one can push the camera.

reference library

Here the reference library is used to investigate a g2v star. This is the same type of star as the sun. The next move would be to click Planck and check that 5700K is the right temperature for this curve.

You can view the solar spectrum by reflecting sunlight from along the length of a needle at the camera. Since all that is required is to view it is a bright source of light lined up along the yellow 0 nm line, it is quite possible to take advantage of this and observe the sun. Most visible in the spectrum will be the g2v black body curve and, if you zoom in, the Fraunhofer Absorption lines.

Perhaps an easier demonstration is to view the clear, blue sky through a slit. This reveals that the blue sky is actually a mixture of all colors with more blue than any other color. The truth is that there is actually a little more violent than the camera can reveal but like the human eye the camera is less sensitive to violet than to blue. This “unsaturated blue” (meaning blue + white) is consistent with the Rayleigh Scattering Model for why the sky is blue.

If you own a telescope you may be able to analyze the spectrum of the stars with the RSpec-Explorer. Because of the sensitivity limitations of the camera, it is not possible to observe stars without a telescope. But, a telescope can help a lot. Be sure to know which star you are looking at (I recommend Sirius, Betelgeuse, and Vega), then look up what type of star they are (a1v, m2i, a0v respectively).

 

Conclusion

spectrum analyzing equipment

Old-fashioned – but not obsolete – spectrum analyzing equipment.

Experiments on light can be very engaging, but they can also be very confusing. It is important that we take steps to ensure that our students are able to view what they are supposed to be seeing, and recognizing what is being pointed out. The RSpec-Explorer projected overhead for your students’ benefit is probably the best way to in engaging your students in spectroscopy (especially if used in conjunction with hand-held spectrum analyzing devices). I have found that students are very interested in cameras and how they can see things that our eyes cannot. If building a community of learners in your science classroom is your goal then you should add this device to your collection of lab equipment.

Projecting Image

Projecting this image for your class will ensure that they will understand what you mean when you say, “Yellow light is dispersed further than violet light, and red light is dispersed the most.”

 


James Lincoln

Tarbut V’ Torah High School

Irvine, CA, USA

James Lincoln teaches Physics in Southern California and has won several science video contests and worked on various projects in the past few years.  James has consulted on TV’s “The Big Bang Theory” and WebTV’s “This vs. That” and the UCLA Physics Video Project.

Contact: [email protected]


 

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$100 close up

Turn Your Mobile Phone into a Mobile Microscope [W/Video]

Can the new Cell Phone Magnifier unlock why Mint Mentos are better than Fruit Mentos for making soda explode? Check out this cheap option for those times when high-powered microscopes aren’t necessary.

Arbor Scientific has a marvelous Cell Phone Magnifier that can easily be attached to a cell phone camera and turn it into a microscope.

You can use it anywhere you want, any time you want; for example you can investigate something on your nature walks that would be interesting microscopically such as an insect or plant. But I have found a lot of use for it in my lab when I view or photograph something microscopic. In this article I will provide some examples of interesting objects to view and discuss some details of the physics of the lens.

Examples of Interesting Objects
For starters, here is the surface of an ordinary piece of printer paper. This shows quite easily that it is not smooth at all. This is important for explaining why paper does not reflect light the same way a mirror does (diffuse vs. specular reflection).
paper
mentos-1 Here are up-close pictures of Mentos candies.

Can the microscope unlock their secret powers of soda exploding?

mint

The Mint Mentos display a pitted surface.

fruit

 

The Fruit Mentos have a glaze that inhibits nucleation sites.

You can clearly see the dimpled “nucleation sites” that are the catalysts for releasing the bubbles from soda. The Fruit Mentos is more glazed over and does not work as well as the Mint Mentos.

The new $100 Bill is an excellent candidate for this tool.  Many of its security features are only appreciably if you have a microscope.  For example, the tiny security writing along the length of the feather, or the security threads that are thrown in to curb counterfeiting.  Also, note that you can clearly see the color changing glitter is lain in different directions on each side.  Shine a flashlight at the bill for best results.  Don’t miss the raised ink on the front of the bill, you can feel it with your finger, but only with a microscope can you recognize the very specific patterns that in the relief.

100

Illumination from the side helps the features of the new $100 Bill to stand out.

 100close

The color change glitter effect comes from having different colors on each side. 

100rased

Raised ink and holograms are among the new features.  Note the pattern is the Liberty Bell. 

onehundredusa

A security fiber and the micro printing are best viewed through a microscope lens. 

One of my favorite experiments is to hold it up to a computer screen and see the different pixels that are making the colors.  The lens offers an excellent opportunity to study how the primary colors of light are combined to generate new colors.  When looking at pixels, I recommend the Google.com logo.

 

google

This summarizes what you will take away from viewing the Google logo microscopically. 

You will be using a computer screen anyway and it is such a familiar sight that students find this investigation highly amusing.  Also look at the black cursor with a white background, to learn how white light is made.  While you are at google.com you might as well do an image search for specific colors, like pink, or brown and find out how they are made by mixing RGB.

 

Speaking of mixing colors, take a close look at an image in a text book and see that these pictures are not made from RGB, but rather the primary colors of ink, CYMK.  Cyan, yellow, magenta, and black are unequally mixed to form the color desired.  For example, cyan and yellow makes green.  That they are not red, yellow, and blue, is sometimes a surprise.  Note also how messy individual letters look close up.

CMYK image

A CYMK print image of a red car, green bushes, and a blue sky. 

Pretty much any grain or crystal will look very interesting microscopically.  Take a closer look at your rock collection or – even easier – just take some condiment packets from your local restaurant.

salt, sugar, pepper, and sweet'n low

Some good things to image that you can probably get for free.

salt close

To hold the world in a grain of sand.

Pepper close

Of course insects offer a wonderful venue for this lens.  Because the camera is mobile, you now can look at and photograph living insects more easily.  Nonetheless, I still enjoyed looking at my insect collection with the camera.

wasp close up

A wasp from my students’ collections.

fly close up

A flesh fly from the same collection. 

The Physics of the Lens

Using your cell phone as a microscope has advantages beyond just the fact that it helps you make and share videos and pictures.  One of the best things is that your camera has an autofocus, which makes it easier to get the image.  Also, since the lens is making a virtual image, you do not have to move the object oppositely to the direction you want the image to move (this is a major annoyance when using conventional microscopes).

The lens has a focal length of about 1cm.  I found this by projecting a real image of my ceiling lights and by assuming that since the object distance is so much greater than the image distance, that the focal length is the same as the image distance ( 1/f  = 1/do +  1/di ).

 

lens

When the image distance is much smaller than the object distance, the focal length is very nearly the same as the image distance. 

Our website (Arborsci.com) says that the magnifying power is 15 times.  I found this to be true.  I first took a picture of an ordinary meter stick without the lens and found that my iPhone’s lens had a magnification of 2.  Then I took a picture of a ruler with and without the lens, and compared those.  Here I found the increase to be a little more than 7 times.  (Images must be in focus.)  The product of 2 and 7.5 is 15 which is very the same as the advertised value.  I am using the definition that magnification is the ratio of the image height to the object height.  Since our images are not upside down we have a +15.

 

Since the lens needs to be sticky to stick to the cellphone, it also is sticky to dust.  This is easily washed off however, either by a little water or more conveniently and effectively saliva.  The instructions recommend water with a little soap, but water alone is usually sufficient.

 

Conclusion

I am surprised at how much I have come to like having this little attachment.  I have been using it whenever I need to show my students things that are better appreciated through a microscope.

 

With the advent of every student having a camera in their pocket, it would certainly not be an unjustifiable purchase to buy a class set of these to increase engagement when learning about lenses or any subject that uses microscopes.  If you are interested, you should definitely buy one to try it out; you’ll be glad you did.

 


James Lincoln
Tarbut V’ Torah High School
Irvine, CA, USA

James Lincoln teaches Physics in Southern California and has won several science video contests and worked on various projects in the past few years.  James has consulted on TV’s “The Big Bang Theory” and WebTV’s “This vs. That”  and  the UCLA Physics Video Project.

Contact: [email protected]


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Sidewalk Art of Julian Beever

The great artwork of Julian Beever is a side bar to the Arbor Scientific CoolStuff Newsletter on Science in Art. If you find this as fascinating as we did you might want to see the Shadows in Science and Art article.

Julian Beever before

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

artwork of Julian Beever

How it’s done…..

artwork of Julian Beever

From the front looking down the sidewalk with the Artist standing at the end of the image.

artwork of Julian Beever

Looking from the side of the walk gives a completely different perspective on the actual image.

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Polarization

You may not be aware of it, but polarized light is all around you. Do you have an LCD flat screen television, a laptop computer, a calculator, or other liquid crystal display? Well then, you have been exposed to polarized light. Light reflected from non-metallic surfaces such as water, a polished table top, and glass is also partially polarized as is the light scattered by the earth’s atmosphere.

Generally speaking, the human visual system is not particularly adept at perceiving polarized light. While some animals, bees in particular, are quite sensitive to polarized light, humans usually find it difficult to detect it with the unaided eye. (see Haidinger’s brush in Interesting Links)If you are not familiar with the properties of polarized light, then you may have some questions such as…What exactly is polarized light? How does light become polarized? What are some practical uses of polarized light?

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Some Background Information on Polarization
Shaking the end of a Slinky up and down will produce a train of crests and troughs. As the wave moves down the Slinky, all segments of the spring will eventually be set into vibration in a vertical plane (see figure). If the Slinky is shaken side to side, or for that matter in any one direction, once again waves vibrating in a single direction will be produced. These vibrations are said to be plane polarized.

Light emanating from a common source, such as a candle, light bulb, or the sun, consists of vibrating electric and magnetic fields that are randomly oriented in the plane perpendicular to the direction in which the wave is traveling. Such light is unpolarized (see left side of the grid in the figure below). The light shown to the right of the grid is plane polarized in the vertical direction.

Polarization by Selective Absorption
Key Concept: Some materials selectively absorb light with the electric field vibrations in a certain direction but pass light with electric field vibrations perpendicular to this direction. The mineral tourmaline is an example of such a material. A well-know, synthetic polarizing material is Polaroid. An ideal Polaroid filter should absorb 50 percent of incident light.

Try This: Observe a source of light such as a light bulb through a Polaroid filter. What effect does the filter have on the light passing through it? Observe what happens as you rotate the filter. Do you detect any change in intensity of the light viewed as the filter rotates? If you have an LCD screen available, observe what happens when you view the screen while rotating the filter.

Crossed Polarizers

Key Concept:
As the figure shows, a transverse wave on a rope will pass through both fences when the slots in the fences are aligned, but will be blocked when the slots are at right angles to each other.

When light passes through a device called a polarizer, such as a Polaroid filter, only the waves vibrating in one direction pass through; all other light waves are absorbed. When a second polarizer, often referred to as an analyzer, is placed over the first, and slowly rotated, it is possible to totally block the light. Just as with the fence analogy, a wave that passes through one polarizer is absorbed by the second polarizer (see figure). This occurs because the transmission axes of the polarizers are “crossed,” that is, at right angles to each other.

Try This:
Overlap two sheets of Polaroid material. Look at a source of light through the filters while rotating one of the filters. Describe what you see. When the light is completely blocked by the overlapping filters, rotate them through 90 degrees. What do you observe now? Rotate the filters through an additional 90 degrees. Explain what is happening as you rotate the filters.

Bogus Barrier
Key Concept:
Two polarizing filters with their polarizing axes crossed, that is, at right angles, will pass no light.

Setup Instructions:
Cut a wide “window” in opposite sides of a shoebox. Cut a piece of polarizing film (Polaroid) into four pieces, keeping track of the directions of polarization. Attach the film to the windows as shown, so that the two pieces in each window are polarized differently, but films directly across on the opposite window are polarized alike. Place a small ball in the box. Replace the lid. Provide another piece of polarizing film at the station for student investigation.

Try This:
Examine the inside of the shoe box by looking through the tinted windows on either side. DO NOT OPEN THE BOX! Note that a wall divides the inside of the box into two regions. Tilt the box so that the ball rolls back and forth. Can you explain this mysterious behavior?


Notice the direction of the polarized film panels.

If you are totally baffled, you may take the lid off the box. To discover why the “bogus barrier” exists, look through each of the windows with one of the square Polaroid filters provided at this station. You may find rotating the filter while looking through each window quite revealing.

Polarization by Reflection

Key Concept:
When light is reflected from a non-metallic surface, such as a tabletop, snow, or water, it becomes polarized in a plane parallel to the surface. In Polaroid sunglasses, the axes of polarization are vertical. The reflected glare, which is at least partially polarized in the horizontal direction, is reduced by the Polaroid sunglasses.

Try This:
Find a surface (e.g., tabletop, blackboard, etc.) from which the reflected light produces glare. While viewing the glare through a Polaroid filter, rotate the filter until you no longer see the reflected light. In this position, the filter’s axis of polarization is vertical. Place a small piece of masking tape along the edge of the filter and indicate the axis of polarization with an arrow. Rotate the filter to pass the maximum amount of light. Describe the orientation of the axis of polarization now.

Now view reflected light from a sheet of metal such as a piece of aluminum foil. Describe what happens this time as you rotate the filter. Compare the reflected light from a metallic surface to that reflected from a non-metallic surface.

Getting Quantitative: Finding Brewster’s Angle
View the glare from a surface through a Polaroid filter held close to one eye. When the axis of polarization of the filter is vertical, you will notice that the reflected light is dim for a variety of viewing angles but completely dark for only one. The angle of reflection that produces completely polarized light is called Brewster’s angle and depends on the reflecting surface.

Measure Brewster’s angle for one or two different reflecting surfaces. To do this, have your partner extend a string from the filter to the spot on the surface where the light is totally extinguished. Use a protractor to determine the angle formed by a normal (perpendicular line) to the surface and the taut string.

Brewster’s angle for water is approximately 53 degrees; for glass, 56 degrees. You may wish to find Brewster’s angle for plastic or floor wax by using them as your reflecting surfaces.

Polarization by Scattering

Key Concept:
When light interacts with an object whose size is comparable to the wavelength of light, it shakes the charges in the object. These charges then radiate in all directions. This phenomenon is called scattering. The light filling the daytime sky is due to scattering that occurs as sunlight passes through the atmosphere. Blue light is scattered more than red light. That is why the sky is blue. Atmospheric scattering also polarizes light. Looking at the sky with a Polaroid filter will convince you of this. The figure below illustrates the scattering and polarization processes.

Try This:
If the weather permits, go outside and investigate skylight with a Polaroid filter. DO NOT LOOK DIRECTLY AT THE SUN! Slowly rotate the filter as you view a portion of the sky.

Is it possible to reduce the brightness of the sky for certain orientations of the filter?
Now examine other areas of the sky. Does the light in certain portions of the sky seem to be more polarized than others?

Estimate the angle formed by imaginary lines drawn between your head and the sun and your head and the portion of the sky with the highest degree of polarization.

Look at the portion of the sky with the greatest polarization. If clouds are present in this region, observe what happens as you rotate your filter while viewing the clouds.

Do the clouds seem to stand out for certain orientations of the filter?

This occurs because the light scattered by the atmosphere is polarized, but the light scattered many times by water droplets in the cloud is not.

Simulating Atmospheric Scattering

Atmospheric scattering may be simulated by adding a few drops of milk to a container of water. This simulation works because the solid particles in milk are much smaller than the wavelength of visible light. So if you can’t go outside, you may wish to try performing the following demonstration of scattering.

First fill a transparent container with water. Shine a flashlight beam through the water and observe the scattered beam from the side of the container. Also observe the transmitted beam by projecting it onto a piece of paper. Describe the scattered and transmitted light.

Now add a little milk to the water, a drop or two at a time. Carefully note the color of the mixture as the milk is stirred into the water. What color is the scattered light now? Note also the color of the transmitted light that is projected on the paper. What color do you observe on the paper? Continue adding milk in small amounts and notice changes in the scattered and transmitted light. Describe these changes. View the blue light scattered by the milk particles through a Polaroid filter. What do you observe as you slowly rotate the filter? Can you explain your observation? Also look at the top of the water through the rotating filter. What do you observe?

Finally, use a Polaroid filter to examine the transmitted beam. Is this light polarized? Additionally, place the polarizing filter between the flashlight and the milky water. Look from the side and notice what happens when you rotate the polarizer. Also notice what happens to the transmitted red “sunset.”

Birefringence

Key Concept:
Crystals, such as calcite and quartz, are said to be birefringent, meaning two indices of refraction. When unpolarized light enters a birefringent material, it divides into two components.  These components have different speeds and are polarized at right angles to each other (see below).


Figure 1

Using a Polaroid filter to view the two rays as they emerge from the crystal, either ray can be extinguished while the other remains visible. Students are fascinated by the formation of a double image when an object is viewed through a birefringent crystal (see figure 2).


Try This:
Place a calcite crystal on some printed material. How many images do you see? Now view the printed material after you have placed a Polaroid filter on top of the crystal.

What do you see now? Can you extinguish one image at a time by rotating the filter? Why do you think this occurs?

When stressed, plastic and glass become birefringent. Viewed between crossed Polaroid filters, this birefringence appears as colored contours. Place a plastic fork, or other plastic object, between your filters to make the stress lines visible. If you are using a fork, squeeze the tines together.
What happens to the colored stress lines?

Due to their birefringent nature, some transparent tapes produce brilliant colors when viewed between polarizing filters. Using only transparent tape and a pair of polarizing filters, it’s possible to create beautiful colored designs reminiscent of cubist art and stained glass windows. (Note: Not all clear tapes (e.g. Magic Tape) are birefringent. You may wish to try transparent packing tape or old fashion cellophane tape.)


Did you know You can share the science of Polarization with the art department!
How would you like your students to produce polarization art in your classroom? I can say from experience that it’s an activity capable of engaging your students beyond your wildest dreams! To make the experience even more meaningful, consider collaborating with your school’s art department. At New Trier High School an art teacher and I would often bring art and science students together to introduce them to polarization, color, and artistic composition.

After students perform exploratory activities relating to polarization and color, the art teacher can give them an overview of what makes for interesting composition. Students then set to work applying this knowledge to create their works of polarization art.

They begin the process by layering tape on microscope slides in a step-like fashion. Placing the layered strips between crossed Polaroid filters helps them determine the exact relationship between color and tape thickness.

Petri dish art
Once they have created their color keys, students produced polarization art by placing carefully cut pieces of birefringent tape, such as clear packing tape, on a clear substrate (a sheet of overhead transparency material or a glass or plastic Petri dish works well).

When sandwiched between Polaroid filters, the resulting creations have the appearance of luminous stained glass.

Trick of the Trade: Students can use an laptop LCD screen as a source of polarized light. Placing their art work between the screen and a Polaroid filter will enable them to see their work as it evolves. Taking a cue from Austine Wood Comarow, students can don Polaroid glasses, thereby freeing their hands. 

Optically Active Substances

Key Concept:
Optically active materials are capable of rotating the plane of polarization of transmitted light. These materials include sugar solutions, corn syrup, turpentine, amino acids, and some crystals. When polarized white light passes through an optically active liquid, each color’s direction of polarization is changed by a different amount. Changes in color are observed when the light transmitted through the liquid is viewed through a rotating polarizing filter.

The degree of rotation of the plane of polarization depends on the depth of the liquid. Therefore, different depths of solutions will exhibit different colors when viewed through a stationary polarizing filter. In the photo below, pieces of glass placed in Karo syrup create a variety of depths, and hence different colors.

The Barber Pole Demo

Key Concept:
Molecular antennae, called dipoles, constituting an optically active liquid absorb and reradiate light. This process is the result of electric field vibrations acting on electrons within the molecules. Known as scattering, re-radiation occurs most strongly in the plane perpendicular to each dipole. As polarized, monochromatic light passes through an optically active liquid, its plane of polarization rotates, and with it, the direction of scattering. The figure below shows how the plane of polarization “corkscrews” as it passes through the optically active liquid.

When polarized white light passes through an optically active liquid, the plane of polarization of each of its constituent colors changes by a different amount. Thus each color is scattered in a different direction, producing effect shown in the photo below.

Interesting Links:

Extreme Education: Polarizing Your Face

Birefringence: Polarization

Polarization Light Demo

Polarized light in art

Rainbow The Spiral Link

Haidinger’s Brush Link

Read more...

Atmospheric Optics: She comes in colors…

The sky offers a wide variety of stunning optical effects. A source of inspiration for poets and songwriters alike, these atmospheric phenomena include red sunsets, rainbows, mirages, halos, glories, and coronas. These effects are the result of the interaction of light from the sun or moon with the gases in the atmosphere, clouds, ice crystals, smoke, dust and other airborne particulates. Some of these phenomena can be seen almost every day; others occur less frequently. In this issue of CoolStuff we will examine examples of atmospheric optical phenomena and how they may be demonstrated in the classroom.

The sky is the daily bread of the eyes.
– Ralph Waldo Emerson
 

She comes in colors everywhere;
She combs her hair
She’s like a rainbow
Coming colors in the air
Oh, everywhere
She comes in colors…
She’s like a rainbow

– Mick Jagger / Keith Richards

Concept: 
A rainbow is a multicolored, circular band of light. The display of colors is due to refraction and internal reflection occurring in raindrops or other droplets of water.

Making Your Own Rainbow I: 
Direct a fine spray from a garden hose in a direction away from the sun. How far away do you estimate the rainbow to be? If you do this experiment with a group of people, does everyone see the same rainbow? Do you see your shadow? Where is it located in relation to the rainbow? If you want to explore further, stand on a ladder while producing your rainbow. Describe the rainbow you see now.
Making Your Own Rainbow II: 
In a darkened room, place a clear (the clearer, the better) plastic box approximately three-quarters full of water on the stage of an overhead projector. (Note: These boxes are the type often used to store shoes.) Cover or remove the projector’s top lens so that no light is projected into the room. Arrange the water-filled box so that students can see both of the rainbows formed (a rainbow is produced by each long side of the box.) Examine the array of colors produced by the water-filled plastic box. Are the rainbow colors in the same order as in a naturally-occurring rainbow?

Making Your Own Rainbow III: 
Shine light from the bright flashlight or a slide projector through a central hole in a piece of white cardboard. If a water-filled flask is illuminated with the light passing through the hole (see figure) a faint rainbow will appear on the cardboard. It has the shape of a closed circle and its angular distance is about 42 degrees, with red on the outside, as in a naturally occurring rainbow. You will need a completely dark room since the rainbow formed is quite faint.
Spectrum demonstration:
Discussions on rainbows and the optics of the sky always lead to the topic of the electromagnetic spectrum.

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Another great classroom tool is the Giant Prism. Use it on your overhead projector to project a large class-size rainbow!

Giant Prism

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Hiroto Ashikaga; Tottori Technical High School, Syozan 111, Tottori 689-1103 Japan

Making Your Own Rainbow IV: Tiny glass beads, such as those used by your local highway department to make highway signs and street markings highly reflective, may be used to produce rainbows like those seen in the center photo below. The beads, behaving like raindrops, work in concert to form a rainbow.

Most highway and public works departments will gladly give you a container of glass beads. Once you have obtained the beads, cover a piece of black foam core or poster board with a thin layer of spray glue. Now sprinkle the glass beads over the black surface until the surface is completely covered with beads. When a point source of light, such as a Maglite with reflector removed, is used to illuminate the beads, the beads will form a circular rainbow that seems to hover above the cardboard.

Concept: Blue light interacting with molecules in the atmosphere is absorbed and reradiated in all directions. Blue light is scattered much more efficiently than light with longer wavelengths, for example, red and green. As a result of scattering, the sky looks blue no matter where we look. By contrast, to an observer on the moon, the lunar sky appears black because there is no atmosphere to scatter light.

During sunrise and sunset, the distance that light travels from the Sun to an observer on Earth is at its greatest. This means that a large amount of blue light and some green light is scattered. Since white sunlight may be thought of as consisting primarily of blue, green and red light, the blue/green deficient light that we see coming directly from the sun appears red. 

Blue Sky/Sunset Simulation I: One of the most frequently asked questions is “why is the sky blue?” Using very simple equipment, you can demonstrate and explain the phenomenon to your students. Add a few drops of milk or a few grains of powdered milk to water in a beaker or fish tank and stir. The milk particles serve as scatterers just as air molecules do in the atmosphere. When light from a light bulb or slide projector passes through the liquid, scattered blue light may be seen throughout the container.

Shine light from a light bulb or slide projector through the liquid and observe the color of the transmitted light. With much of the blue light removed from the incident white light by scattering, only the orange-red portion of the spectrum remains. When viewed head on through the liquid, the transmitted light actually looks like a setting sun!

If you are using a slide projector and fish tank for the simulation, you may wish to carefully rotate the tank as it is illuminated. Allowing light to first pass through the narrow width, then through the length of the tank, allows students to observe how the color of the sun changes from a yellow-orange to an orange-red as it moves from its noon day position to the horizon.

Blue Sky/Sunset Simulation II: A second method of demonstrating why the sky is blue and the sunset red requires the use of two common chemical substances: dilute sulfuric acid (H2SO4) and sodium thiosulfate (Na2S2O3), hypo used in photography to fix developed films. (Caution: be careful when handling the acid.) First mix three teaspoons of thiosulfate with one liter of water. To this solution add ten to twelve drops of acid. After a few seconds, the solution will take on a bluish tint. With time the color will become more intense, then fainter. After a few minutes the liquid will turn white.

These changes are due to the scattering of white light from tiny grains of sulfur which gradually grow in size as the reaction progresses. Initially, the grains are very small and serve as scattering centers for short wavelengths of light, hence the blue color. Eventually the particles become so large that they scatter all wavelengths of visible light with equal intensity. This accounts for the final milky appearance of the liquid.


Note that a cardboard mask blocks the light not passing through the beaker.

Scattering from particles whose dimensions are much less than the wavelength of light is known as Rayleigh (pron. ray-lee) scattering. Rayleigh scattering is responsible for the blue appearance of the Earth’s sky. The non-preferential scattering by larger particles is known as Mie (pron.me) Scattering and is responsible to the white color of clouds.

A beautiful setting sun effect can be achieved by placing a beaker containing the H2SO4 – Na2S2O3 solution on the stage of an overhead projector (see image left). First mix three teaspoons of thiosulfate with one liter of water. To this solution add ten to twelve drops of acid. (Caution: be careful when handling the acid.)

A mirror is used to project the light passing through the beaker onto a screen. As the sulfur particles grow in size, the scattered blue light will become more intense while the light reaching the screen will change from white, to yellow, to orange and finally to a deep red.

The Color of Clouds

Concept: Clouds consist of water droplets and ice crystals that are significantly larger than the wavelengths of visible light. Unlike the smaller gas molecules that make up the Earth’s atmosphere, these larger particles scatter all colors more or less equally.

Looking at a cloud, an observer will, in most cases, receive all wavelengths of light and perceive it as white. However, a cloud’s actual appearance is governed by color of illuminating light, cloud thickness, shadowing by other clouds, age of the cloud, and the brightness of surrounding sky and clouds. Thicker clouds transmit little light and hence may appear darker. Larger droplets in older clouds scatter less and absorb more light than smaller drops and therefore appear darker.
The Whitest Cloud Around:
What we identify as white is simply the brightest gray in sight. A light gray cloud on a bright white background will look much darker than the same cloud on a dark or black background, in which case it might look white and bright. To demonstrate this, obtain a variety of paper samples, each of which appears to be white in isolation. Place them side by side, or cut them so that they can be nested on top of one another, for comparison. Usually only one will be perceived as white; the other samples will appear gray by comparison, as it is with clouds.
Concept: 
A halo is an optical phenomenon due to reflection and refraction of sunlight or moonlight in atmospheric hexagonal ice-crystals. Halos appear as bright rings around the sun or moon. Although they are more common in cold weather, halo-producing cirrus clouds can be present in warm weather. Colored halos are formed by refraction in the crystal; white halos are produced mainly by reflection. (see below left)
Free Download! Double-Slit Diffraction
With just a Laser Pointer and a Laser Printer each of your students can now generate their own double-slit patterns — and it’s FREE!  Click here
Produced by irregularly-sized droplets, these coronal fragments appear as wisps of iridescent pastel colors in clouds.

Cool Coronae:
To produce a corona, simply breathe on a cool piece of glass. More often than not, a corona will be seen by looking at a light source through the water droplets that condense on the glass. If you wear eyeglasses, simply exhale on one of the lenses. When you look at a light source through the lens you will see a corona whose colors change with time. Since the colors produced depend on droplet size, the colors change as the droplets get smaller and finally disappear.

You may not even have to breathe on glass to observe coronae. You may see them through a fogged windshield or on steamed up glass in the bathroom.

Iridescent Cloud in a Bottle: 
Iridescent coronae are often produced by the water droplets that make up thin clouds. So to produce coronae it would seem that all you need to do is make a cloud. Using a gallon jar, a rubber glove, some water and a match, you can do just that. First cover the bottom of the jar with a thin layer of water. Drop a lit match into the jar. Quickly place the fingers of the glove inside the jar and stretch the open end of the glove over the mouth of the jar. Put your fingers the glove and pull the glove outside the jar. Presto! You should see a wispy cloud inside the jar.

To observe a corona, shine light from a bright source such as a slide projector or flashlight through the jar. Initially smoke particles will scatter all wavelengths of light producing a white cloud. As the smoke disappears, leaving smaller droplets, pastel colors will be seen at certain viewing angles. You’ve just observed your first corona in a bottle!

The figure above shows a rare atmospheric optical phenomenon known as a circumhorizontal arc. Caused by the refraction of light through the ice crystals in cirrus clouds, it occurs only when the sun is high in the sky, at least 58° above the horizon.

Reminiscent of a rainbow, the circumhorizontal arc is produced only when the ice crystals making up cirrus clouds are shaped like thick plates. Furthermore, these plates must have their faces parallel to the ground. The chances of having all these conditions satisfied are low, hence the infrequent observation of this amazing optical phenomenon.

Other Cool Sky Stuff

In the photo shown here, the Aurora Australis is seen over the National Science Foundation’s Amundsen-Scott Pole Station.


Aurora Borealis

The Northern and Southern Lights, or more formally Aurora Borealis and Aurora Australis respectively, are produced when charged particles from the Sun pass through the Earth’s upper atmosphere. The high-speed particles energize gas molecules which in turn emit the ephemeral colored lights we associate with the Aurora.

This image is courtesy of UK photographer Rich Lacey. While spending time in Northern Canada Rich had to opportunity to capture the best Aurora photos we’ve seen. You can see more of his images and order prints for your class on his web site at www.richlacey.com

Light Pillars

Often seen in very cold weather, light pillars seem to be beaming up from terrestrial light sources such as street lamps. Many initially mistake light pillars to be searchlights. Light pillars result from the reflection of light from hexagonally-shaped, plate-like crystals. These crystals fall with their flat surfaces in a horizontal orientation. The flat surfaces serve as mirrors, reflecting the sun’s light downward.

A sun pillar is a vertical shaft of light extending upward or downward from the sun. Like light pillars, they are produced when sunlight reflects off the surfaces of plate-like ice crystals. Sun pillars are usually seen at sunrise or sunset when the sun is low on the horizon.

Interesting Links:

Northern Lights! http://www.northern-lights.no/

About Rainbows http://eo.ucar.edu/rainbows/

Atmospheric Optics http://www.atoptics.co.uk/

Halos http://www.polarization.com/rainbow/rainbow2.html

Weather Optics http://www.aws.com/aws_2001/schools/wx_mania/063003.htm

Read more...

Hallway Science: The Science Experience

Science is too interesting to keep it cloistered in the classroom! For almost 30 years, we have been sharing the wonders of nature with others in our school through the use of display cases and exhibits located outside the classroom. While doing science in the hallway or other non-traditional settings may seem a bit unorthodox, this form of informal education beckons members of the entire school community to learn just how interesting and enjoyable science can be.

Science displays are always a favorite of parents attending a school’s open house. Adults are always interested in seeing what goes on in their son or daughter’s school, and science displays provide one window into the school’s academic program. Parents often comment that the science displays are the highlight of the open house.

Our first involvement in hallway science displays occurred when we realized that the primary use of our school’s display cases was to store and display trophies. Many cases were not used at all. It occurred to us that we might be able to use these showcases as extensions of the classroom. Our very first effort revealed that a science showcase attracts both science students and non-students alike. In fact, we learned that the display case is an excellent way of introducing non-science students to the wonders of the various scientific disciplines. Students often spend their passing periods trying to understand some phenomenon that to them seems paradoxical or a violation of common sense.

Interactivity is the key to a good display. Doing, not just looking and reading, engages both hands and minds. Furthermore, the more open-ended an activity, the better. Individuals should be able to view the apparatus as a vehicle of discovery and feel free to ask, “what will happen if I do this or that.” As you’ll see, even displays behind glass can be interactive.

The following examples of hallway exhibits and display cases have been very popular with our students. While inexpensive and simple to build and use, they have provided hundreds of students of all ages with a great deal of pleasure and perhaps a desire to learn more about the wonderful world in which they live. Hopefully, these exhibits will get you thinking about ways of decking your halls with science!

The Art of Good Science Displays

Polarization Tape Art Display Case

Some transparent tapes separate white light into its component colors when sandwiched between two polarizing filters. Using clear packing tape and a pair of polarizing filters, your students can create beautiful colored designs reminiscent of cubist art and stained glass windows.

The colors produced depend on the thickness of the tape. Tape is cut into desired shapes and layered on a transparent substrate such as a blank overhead transparency. By varying the number of tape layers in each region, a full palate of color is available to the budding Picassos.

In a recent display case, student-produced tape designs were displayed on a light table borrowed from the art department. The tape art was placed on a sheet of polarizing film that covered the stage of the light table. When visitors viewed the artwork through a hand-held Polaroid filter (available outside the display case, loose or tethered with string) brilliant colors were observed. Rotating the polarizing filter produced dramatic changes in the observed colors. Oohs and aahs were frequently heard coming from passers by who stopped to view the display.

Making polarization tape art may be used as a culminating activity after studying light and color in physics class or as an inter-disciplinary project. For example, we brought physics and art students together for a week so that the “two cultures” could gain both a knowledge and appreciation of what are usually considered to be disparate disciplines. Sharing the finished artwork through a hallway exhibit allows all to enjoy the marriage of art and science.


Light will pass through two polarizing filters with their axes of polarization aligned. However, when two polarizing filters have their polarizing axes “crossed” (i.e., at right angles), no light will be transmitted.

Polarizing Film Sheet 30cm x 38cm

In Stock SKU: P2-9415
$59.00
Visual Perception and Illusions Display Case


Einstein Alive

In Stock SKU: P2-6000
$20.00

The possibilities for designing a display case on visual perception are endless! Perhaps the simplest approach is to use printed illusions. Figures and photos of visual illusions found in books may be photocopied. Engaging posters may be purchased in both shops and on the Internet. Suggestions for viewing the images along with brief explanations of the illusions are recommended. While not physically interactive, a display of illusory images has the power to engage and amaze.

Simple three-dimensional exhibits may also be incorporated into the illusion display case. A collection of reverse masks makes for a great display! We are accustomed to seeing convex faces so it is not surprising that when presented with a concave face, we unconsciously see what we expect to see. The reverse Einstein mask shown here certainly appears to be in relief even though it’s not. But there’s more! When you walk by the face, it appears to follow you. A simple yet effective reverse mask results from viewing the concave side of an inexpensive plastic mask. The effect is often best if the mask is white. Filling a display case with a number of these masks makes for a most eerie exhibit!

Pipes of Pan

A trip to the carpet store was the genesis of the giant ambient noise resonators or Pipes of Pan, as they are sometimes called. Eight carpet tubes mounted on a plywood base became the basis for a rather strange musical instrument. Based on the principle of resonance, the air in each tube vibrates with a frequency determined by the length of the tube. The background noise in a room contains virtually all audible frequencies, and is capable of creating resonant vibrations in each of the tubes.

We simply put our Pipes of Pan in our school’s central hallway and allow people to explore. A sheet with suggestions for use and a brief explanation of the apparatus is provided. Needless to say, the unusual musical instrument is almost always in use.As is seen in the photograph, a person placing their ear near the end of one of the tubes hears a definite pitch. Moving from one tube to the next in succession, the listener hears a musical scale. Some people try to play a simple tune by rapidly jumping from one tube to the next.To make your own Pipes of Pan, ask your local carpet installer for carpet tubes. The carpet tubes should have a combined length of at least 8 m (roughly 24 ft). This length allows for loss that will occur during cutting. The tubes should be cut to the lengths in the chart below. The chart also shows the corresponding note and resonant frequency for each tube. The tubes may be painted (optional) and attached to a sheet of plywood with small bolts. The tubes may also be simply placed on a tabletop with end stops to prevent rolling.


For ready-made sound tubes, check out: 

Set of 8 Boomwhackers

In Stock SKU: P7-7400
$34.00

Multi-dimensional Shadows

This Exploratorium-inspired exhibit is visual ambiguity set in motion. As you stare at the shadow of a slowly rotating cube, you notice that it mysteriously appears to reverse its direction of rotation. A quick check of the actual cube reveals that it motion is unchanging. What gives?Rotational ambiguity arises when the three-dimensional cube is compressed to a two-dimensional projection, removing important visual cues. Finding either direction of rotation equally acceptable, the mind perceives the cube to rotate in one direction, then the other.As the photo indicates, the exhibit is very simple. A cube fashioned from balsa or soda straws is suspended from a slow turning motor. A slide projector is used to form a shadow of the cube on a translucent screen. Our screen is made of muslin. PVC pipe may be used to form the support for the screen and the motor, but ring stands also work quite well. Two ring stands support the muslin screen while a third ring stand and clamp hold up the motor and cube assembly.

This device may be modified slightly for Halloween. Replacing the cube with a dangling plastic skeleton adds an additional creepy element to an already eerie display.

Interactive Bubble Machine

Who doesn’t like to blow bubbles? With the interactive bubble machine, students can blow bubbles of unimaginable proportions. They can also study the beautiful colors produced by thin film interference as well as standing waves on the surface of the film.As the figure shows, the device consists of a PVC frame supported by a wooden base. A PVC rod, attached to a rope that passes over a pulley at the top of the frame, is lowered into a tank of bubble solution. When the horizontal rod is retracted from the solution, a sheet of soap film is produced that fills the space between the upright poles of the frame.The exact dimensions of the frame are not important. The tank, an inexpensive plastic flower box, is filled with a bubble solution that consists of one part Joy or Dawn dishwashing detergent and six parts water (Note: you may wish to experiment with the bubble solution so as to obtain optimal results). Two lengths of fishing line are used to keep the horizontal dipping rod in the plane of the device’s frame. The two lengths of fishing line attach to the top of the frame, an anchor in the tank, and pass through holes drilled at each end of the horizontal rod.We have used the bubble machine in a variety of venues and found it to be one of those things people can’t keep their hands off of. We have placed the device in the hallway, at the back of the classroom, and, with certain modifications, in a display case. Regardless of the setting, everyone feels challenged to produce the largest bubble. As the figure shows, this can often be achieved with two people blowing on the soap film.If the bubble machine is placed in the hallway, the floor can become slippery due to spilled bubble solution. To circumvent this problem, we purchased a rubber mat with holes in its surface that allow for drainage.

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Haunted Laboratory: Halloween Physics

Each October the physics teachers at New Trier High School in Winnetka, Illinois treat their students and the public to a haunted science laboratory. Visitors learn science while having fun as they are confronted with a maze of displays that demonstrate optical, acoustical, mechanical, electrical, and perceptual phenomena in the context of Halloween. Scientists and non-scientists alike become engaged in trying to understand the science behind the fun.

As strange as it may seem, science and Halloween do have something in common: they both exemplify our innate fascination with the mysterious. The concept of a haunted science lab was conceived of over twenty years ago by Creighton University physics professor Tom Zepf. It began as a collection of activities in Light, Color and Lasers, a core-curriculum course he was teaching at Creighton. Gradually the annual Halloween event evolved into laboratory-based experience consisting of over three dozen displays demonstrating a wide range of physical principles, all of which invite interaction.

In the last few years, the concept of integrating science with fright has spread to other educational institutions (e.g., New Trier High School and Deerfield High School, both in Illinois, and Luther College in Iowa). In the process, displays illustrating perceptual concepts have become a part of the annual exhibitions. Like light and color, visual perception lends itself extremely well to the Halloween theme. The inclusion of psychological and physiological phenomena has made the haunted lab relevant to students of psychology and biology while demonstrating connections between the sciences.

We now offer some examples of favorite haunted lab exhibits. The beauty of these displays is that they are based on apparatus found in most science storerooms. With slight modification, many devices commonly used in the traditional science laboratory may be transformed into something spooky.

Student Activities

1. Mirrorly a Ghost

Key Concept

An object placed at the center of curvature of a concave mirror will produce an inverted real image whose size and distance from the mirror exactly match that of the object.

Teacher Instructions

Based on the standard spherical mirror demonstration, a large concave mirror is used to produce a ghostly apparition in an exhibit we like to call “Mirrorly a Ghost.” An illuminated, inverted plastic ghost is mounted at the center of curvature of a large concave mirror. The mirror forms an erect image of the ghost directly above the object ghost. The illusion is made complete with the addition of arms, shirt cuffs, and a white sheet extending from the image to the floor.

Concave Mega Mirror

In Stock SKU: P2-7150-02
$159.00

Image courtesy of New Trier

High School’s Haunted Lab

Student Instructions
Look at the ghost before you. Is it real or is it virtual? Hint: The actual ghost is located just below the ghostly image you see. Hint #2: A concave mirror is used to produce this spooky illusion.

2. Apple Oscillators

Key Concept

When two pendulums have the same length, they will oscillate at the same frequency. Scientists say that the two pendulums exhibit resonance. If the pendulums are coupled in some way, say by a string, the gentle tugging produced by the connecting string will efficiently transfer energy back and forth between the two pendulums.

Teacher Instructions

Two apples suspended from strings and linked by a string or rubber band can be made to stop on command. Knowing that the apples’ energy is transferred back and forth allows the operator to predict when each apple will stop.

 

Student Instructions
Start one apple swinging by pulling it back a few centimeters. Now carefully watch what happens. Notice that the amplitude of swing of the apple you released is decreasing while the apple initially at rest is starting to move. Will the first apple ever stop swinging? You’ll have to watch and see!

3. Pumpkin Pendulum

Key Concept

Conservation of Energy: Energy cannot be created or destroyed, only changed from one form to another.

Teacher’s Instructions

A pendulum with a pumpkin as a bob is used to demonstrate the conservation of energy. The pumpkin is supported from the ceiling with a rope. A volunteer stands with back against the wall. The pumpkin is brought back to the volunteer’s nose and then released. The trick is to remain motionless with your eyes open as the pumpkin first swings away and then returns.

Special Note: Be sure that you make a notice to the pumpkin user not to give it an initial push! This could have a big impact on the participant!

Student Instructions

With your back against the wall, hold the pumpkin so that it just touches the tip of your nose. Now with your eyes wide open, release the pumpkin and wait for it to return. Try not to flinch as the pumpkin comes hurling back at you!

4. Ghostly Apparition

Key Concept

Real images are formed where reflected light rays converge.

 

Teacher’s Instructions

At the heart of this demonstration is the Mirage. The device consists of two inward facing concave mirrors with identical focal lengths. The two mirrors fit together forming a shape reminiscent of a flying saucer. The ghost, placed on the surface of the lower mirror, produces a real image in the plane of a hole cut in the upper mirror.

Student InstructionsGhosts are untouchable and this exhibits proves it! A tiny ghost figurine is seen perched on top of what appears to be two bowls inverted on each other. When you attempt to touch the ghost, your fingers go right through it!
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download the Anamorphic grid
Download Halloween Anamorphic art

5. Spooky Anamorphic Art

Key Concept

Anamorphic images are purposely distorted and require a cylindrical mirror to make them intelligible.

Teacher Instructions

Anamorphic images of ghosts, goblins, and other things that go bump in the night may be produced using computer graphics programs. For example, Print Artist from Sierra allows you to wrap images into the required semicircular shape. This program also has a library containing many Halloween images.

We encourage you to have your students produce their own Anamorphic Halloween images using the grids found at our link. Instruct them to first draw a picture on the rectangular grid. Then tell them to transfer their drawing, point by point, onto the cylindrical grid. As they do so, their image will become distorted. However, it will appear normal when viewed with a cylindrical reflector.

Student Instructions
Place a cylindrical reflector at the center of the distorted image. Look at the image of the distorted image in the cylindrical mirror. What do you see? Does the image now appear normal?
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6. Reverse Masks

Key Concept

The eye-brain system is conditioned to interpret all faces as convex even when they are not. Since we virtually never encounter a concave face, we tend to see what we believe rather than believe what we see.

Teacher Instructions

This reverse mask illusion relies on our expectations. We tend to see what we have learned to see, in this case a convex face. You may wish to have your students experiment with lighting. Suggest that they try illuminating the mask from both top and bottom and from behind if the mask you are using is translucent. We are accustomed to seeing the subtle shadows produced when convex faces are illuminated from above. The same shadowing results when light from below shines on a concave face. Light passing through the mask will produce a similar precept.

Student Instructions
Stand a few feet away from the mask. With one eye closed, look at the mask. Does the mask appear concave or convex? Now open both eyes. Does the mask continue to look convex? Finally, with both eyes open, move from side to side. Can you escape the gaze of the mask?

7. Ectoplasm

Key Concept

Gas molecules that fill the globe are stripped of their electrons by electromagnet waves emitted by a transmitter at the bottom of the globe. An eerie discharge is produced when the electrons recombine with the ionized gas molecules.

Teacher Instructions

You may wish to bring a fluorescent lamp near the globe. The electromagnetic radiation responsible for ionizing the gas molecules with the globe will also excite the phosphors that line a fluorescent lamp. Bring a radio near the globe. You should be able to hear the noise produced by the EM waves emanating from the globe.

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Student Instructions
Change the electrical discharge pattern by gently touching the glass with your fingers. Placing your hand on the sphere may reveal the lines in your palm.

8. Magic Wand

Key Concept

An image is visible when focused light is reflecting off of something. Also, the brain retains images for a fraction of a second in an effect known as Persistence of Vision.

Teacher Instructions

Focus a 35 mm slide of your choice (I like to use a ghost image) on a sheet of white paper hung in the middle of the room. After obtaining a sharp image, remove the paper. Use a tape line on the floor to indicate the position of the paper. Students will rapidly swing a long dowel rod up and down in the area previously occupied by the paper. The dowel will reflect the focused ghost image one bit at a time, and the fast swinging will cause the whole image to persist in the observer’s brain.

Student Instructions
Hold the “Magic Wand” in your hand and wave it rapidly up and down over the tape line on the floor. Look at the side of the wand that faces the projector. What do you see? Wave the wand faster and slower. When can you see the whole image at once? What happens to the image when you stop waving the wand?

9. Ghost Brains

Key Concept

We can see a transparent object when its index of refraction is different than the surrounding material. Ghost Crystals, also called Phantom Crystals, have an index of refraction almost exactly equal to that of plain water.

Teacher Instructions

Combine a tablespoon of Phantom Crystals and 2 liters of water in a large beaker or bowl. Let them soak overnight. The result should look like plain water.

Note: You will need 1 container of phantom Crystals.

Student Instructions
What do you see? Put your hand into the container of Ghost Brains. What do you feel?

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This skeleton rotates only in one direction. However stare at it long enough and what do you see?

10. The Indecisive Skeleton

Key Concept

Two-dimension shadows of three-dimensional rotating objects do not contain enough information for the eye-brain to determine the object’s direction of rotation. Consequently, at any instant, the direction of rotation of the object’s shadow may change!

Teacher Instructions

This exhibit is relatively easy to set up and well worth the effort. A toy skeleton, or any other object for that matter, is attached to the shaft of a slow turning motor. Using a 35 mm slide projector, the shadow of the rotating skeleton is projected on a translucent screen. A blank artists canvas works beautifully.

Student Instructions
Stare at the shadow of the rotating skeleton. You will notice that the skeleton appears to be rotating in one direction, and then, all of a sudden, will appear to rotating in the opposite direction. However, it’s all in your head. The plastic skeleton casting the shadow always rotates in the same direction.

 

 


Don’t miss part 2 of the Haunted Laboratory: Halloween Physics


 

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A Smorgasbord of Optical Phenomena

As we enter the 21st century, perhaps no area of science touches our lives and the lives of our students more directly than optics. We speak on the telephone without realizing that our voices have been converted into digitally-encoded flashes of light that travel over miles of thin optical fibers. Information, whether it be music, images or text, stored digitally on CDs and DVDs is recorded and retrieved optically. At concerts and sporting events, giant screens consisting of thousands of light emitting diodes provide ultra-bright, high-resolution images of figures too small to be seen on the stage or field. State of the art infrared cameras send eerie nighttime images of battles thousands of miles away to our living rooms almost instantaneously. Our understanding of outer space as well as much of the micro-world has been gained through optical exploration. The production of the color images that appear on television, computer, and movie screens relies heavily on optical and perceptual principles. And perhaps most importantly, over 80% of the information we receive from our environment is obtained through the most marvelous optical instrument of all, the eye. The study of light and color has always been the highlight of the year for my students. Optical effects are immediate, captivating, and as we have seen, incredibly relevant to our students’ daily lives. Allowing students to explore with light, and our perception of it, produces a level of engagement that has to be seen to be believed! This collection of laboratory experiences will take your students on a journey through the world of light, color and perception. The learning cycle approach will be employed to allow your students direct experience with optical phenomena and visual perceptual mechanisms. By way of hands-on experiments, your students will learn how light may be beamed, blocked, bounced, bent and even “stored.” They will find out how soot can be transformed into silver and why the myriad colors on your television screen are really just in your head. Through some visual foolery, they will experience the perceptual paradoxes that occur when the brain is unable to make sense of sensation. And in the end, find that they have learned a new way of “seeing the light.”

Optical Phenomena Smorgasbord

Key Concept: The slow emission of stored energy in the form of visible light is called phosphorescence. “Glow in the Dark” phosphorescent materials with long decay times are used to cause watch dials, safety markers, and many children’s toys to glow long after the lights go out.

Setup Instructions:
In a darkened room, provide students with any camera flash and a sheet of Phosphor Glow Paper. If a small fan is available, students may “freeze” the motion of the fan blades by placing the fan between the Glow Paper and the camera flash.

Station Instructions:
Turn on the flash unit. Place your hand or other object (keys, pen, etc…) on the shadow screen. Direct the flash toward the shadow screen and activate the flash. Remove the object from the screen.

What do you see on the screen?

How do explain this image?

What happens to the image after a period of time?

Why does this happen?

Have you seen this material anywhere else?

Images courtesy of New Trier Connections Project, New Trier High School, Winnetka, Illinois.

The picture above shows a person creating a shadow on a phosphorescent screen lit by black light by placing his hand on the screen.

This picture shows the image left on the screen after the hand is removed.

Station 2: Concave/Convex Reflector

Key Concept:
Curved reflecting surfaces are capable of producing both real and virtual images.

Setup Instructions:
Hang a colored ball from the Virtual Reality Mirror so that the ball hangs in the center of the mirror.

Station Instructions:
Pull the colored ball attached to the silvered hemisphere towards you and release it. Watch the images produced by the concave reflecting surface. Describe the changes in the image as the ball swings back and forth and side to side.

Stop the ball and allow it to hang straight down. Where is the image now?

How does the size of the image of the ball compare to the size of the ball?

Look at your reflection in the convex (back) side of the hemisphere. How do you look?

Now hold your finger or other object in front of the convex surface. Describe the image formed. For example, is it right side up or upside down? Is it larger or smaller than the actual object?

Give some everyday uses for convex mirrors.

Station 3:  Over the Rainbow

Key Concept:
As white light passes from one material into another, the constituent wavelengths (colors) travel at different speeds, sometimes causing the white light to break up into a spectrum of colors.

Setup Instructions:
Place a clear plastic (the clearer, the better) box of water on the overhead projector. Close or cover the top lens so that no light is projected forward out of the projector. Arrange the projector so that students can see the spectra that come out of the sides of the box.

Station Instructions:
Examine the array of colors produced by the water-filled plastic shoebox. Can you think of two things that this array of color has in common with an actual rainbow?

List the colors, starting from the inside, present in this rainbow.

How do suppose this rainbow is formed?

How is an actual rainbow formed?

Are colors originally present in white light or is the water the source of the spectral colors in both cases?

Download the anamorphic art grid sheet

Station 4: Anamorphic Art

Key Concept:
Anamorphic images are purposely distorted during their creation and require reflection in a cylindrical or conical mirror to make them intelligible.

Setup Instructions:
Anamorphic images such as these can be found in The Magic Mirror: An Antique Optical Toy, by McLoughlin Brothers. The book is available from major bookstores and includes a piece of Mylar that can be rolled to make a cylindrical mirror. A tail pipe (about 6” long, shiny, straight, with a flanged end) from the plumbing department of the home improvement store, also makes a good cylindrical reflector. Some computer graphics programs will allow you to create your own anamorphic art by printing pictures in an arc.

Station Instructions:
Select one of the distorted works of art. Place a cylindrical reflector at the center of the distorted image.

Examine the reflection of the painting by looking at the side of the cylinder. Describe the image produced by the cylinder. Is the image distorted or does it now appear normal? Use the reflecting cylinder to look at other distorted paintings.

These distorted pictures are examples of an art form known as anamorphosis. A small group of artists began working with anamorphic art during the Renaissance. Anamorphic drawings appear strange and almost unrecognizable to the unaided eye, while a reflecting cylinder reveals an image of normal proportions. This technique is essentially the reverse of a fun-house mirror, which creates a distorted image of a normally proportioned person.

Project:
Use the grids to create your own anamorphic art. Draw a figure of your choice on the square grid, then transfer your drawing, point by point, to the cylindrical grid. Color your work if you wish. To transform your distorted figure into a recognizable object, just look at its reflection in the cylinder!

Station 5: UV Beads (Electromagnetic Spectrum)

Key Concept: Dyes contained in these beads are sensitive to ultraviolet light. When exposed to UV light, they change color.

UV Beads, 250/pk.

In Stock SKU: P3-6500
$5.50

Setup Instructions:
Provide UV Beads, a small UV tube (black light), and other sources of light, including sunlight.

Station Instructions:
Examine the beads and describe their color.

Now place a few beads near the “black light” tube. What happens to the beads when they are exposed to black light?

Can you suggest an explanation for this behavior?

What happens when the beads are now longer exposed to light from the tube?

Hold the beads near a TV screen or computer monitor. If possible, take the beads outdoors so that they are exposed to sunlight. Try other sources of light. Which of these sources of light cause the beads to change color? Why do you think this happens?

Suggest an experiment that you could perform with the beads to test the effectiveness of suntan lotions.

 

Images courtesy of New Trier Connections Project.

Station 6: Vanna

Key Concept:
We not only tend to believe what we see, but we also see what we believe. We are not accustomed to viewing the human face upside down. Consequently we assume, based on past experience, that the inverted face will be normal.

Setup Instructions:
Obtain two identical copies of a photograph. Carefully cut three rectangles around the person’s eyes (including eyebrows) and mouth in one photograph. Glue the photo which is missing the eyes and mouth to the left side of your mounting board. Rotate each of the cut rectangles (containing the eyes and mouth) 180 degrees so that the mouth and eyes are upside down and glue them back onto the photo they were cut from. (This can also be done with a computer.) Glue the uncut photo to the right side of your mounting board. The mounting board can be attached to a slowly rotating motor or some other means for rotating the pictures manually.

Station Instructions:
Watch Vanna as she rotates. Why do you think she changes from gorgeous to gruesome?

Station 7: Bogus Barrier

Key Concept:
Passing light waves through a polarizing filter results in light waves that vibrate in a single plane. Two polarizing filters with their polarizing axis crossed, that is, at right angles, will pass no light.

Setup Instructions:
Cut a wide “window” in opposite sides of a shoebox. Cut a piece of polarizing film into four pieces, keeping track of the directions of polarization. Attach the film to the windows as shown, so that the two pieces in each window are polarized differently, but films directly across on the opposite window are polarized alike. Place a small ball in the box. Replace the lid. Provide another piece of polarizing film at the station for student investigation.

Station Instructions:
Examine the inside of the shoe box by looking through the tinted windows on either side. DO NOT OPEN THE BOX! Note that a wall divides the inside of the box into two regions. Tilt the box so that the ball rolls back and forth.

Does the ball pass through or bounce off the wall?

Can you explain this mysterious behavior?

If you are totally baffled, you may take the lid off the box. To discover why the “bogus barrier” exists, look through each of the windows with one of the square Polaroid filters provided at this station. You may find rotating the filter while looking through each window quite revealing!

Notice the direction of the polarized film panels.
Box drawing is shown without the lid on!

Image shows  a plastic fork, protractor, and an empty cassette case under the polarizing film.

Polarizing Film Sheet 30cm x 38cm

In Stock SKU: P2-9415
$59.00

Station 8 Stressed Out

Key Concept:
Placing certain transparent materials, such as plastic, between two polarizing filters produces colored patterns that are indicative of stress in the material.

Setup Instructions:
Provide two large polarizing filters and several clear plastic objects, such as transparent plastic forks and cassette cases.

Station Instructions:
Place a plastic fork between two polarizing filters. Hold the filter/fork “sandwich” up to the light. What do you see?

How does rotating one of the filters change the appearance of the fork?

Study how stress affects the appearance of the fork by pinching the tines of the fork together as you view the fork between the filters. What changes do you observe?

Now observe other transparent plastic object between the filters.

Polarizing Filters 100mm X 200mm

In Stock SKU: P2-9410
$39.00

Station 9:  Einstein Alive

Key Concept:
With the proper illumination, a concave face may appear convex. This is due in no small part to the expectation we have that human faces are always convex.

Setup Instructions:
Place the Einstein Alive “mask” so that observers will see the concave (inside) side. Light the mask from behind with a light bulb or small desk lamp.

Station Instructions:
Stand about ten feet back from “Einstein Alive”, close one eye, and look at the face. With one eye closed, slowly walk to the left, and then to the right, while you look at the face. Does the face seem to follow you no matter where you go?! Try moving up and down. Can you escape his gaze?

You’ll want to see this link for color addition & subtraction: Color Mixing
http://www.phy.ntnu.edu.tw/java/image/rgbColor.html

Einstein Alive

In Stock SKU: P2-6000
$20.00

Station 10a: Additive Color Mixing

Key Concept:
Three colors of light-generally red, green, and blue- known as the additive primaries can be mixed together to obtain any other desired color.

Setup Instructions:
Cover three gooseneck or flood lamps with red, green, and blue color filters. Attach each lamp to an adjustable power supply. (Adjustable power cords are available at home improvement centers.) Train the beams from the three lamps on a white screen so that they overlap.

Station Instructions:

Adjust the intensity of lamps until their combined effects produce white light. Red, green, and blue are called the additive primary colors. Why?

Block one lamp at a time with a piece of paper and observe the resulting color on the screen. In your own words describe the color produced in each case.

red light + blue light = __________________________

red light + green light = _________________________

green light + blue light = ________________________

The name usually given to the combination of red and blue light is magenta. Red and green light produce yellow. Green and blue light result in what is known as cyan.

To experience the wide variety of hues possible by mixing the three primary colors, slowly adjust the intensity of the lamps. Specifically, try to create the colors listed in the chart below. In each case give a “primary recipe” that others may use to create a particular color. For example, strong red light + medium green light = reddish yellow light.
Colorb”Primary Recipe”

Pink

Orange

Color of your choice

Station 10b: Partitive Mixing

Key Concept:
If small dots of color are placed very close to each other, the eye-brain system will mix these colors additively. This is the basis for color production on a TV screen and computer monitors.

Setup Instructions:
Provide a way for students to place a small drop of water on a computer monitor or TV screen.

Station Instructions:
Place a drop of water on a television screen or computer monitor. Now look closely at the droplet. Careful examination will reveal an array of colored dots. Describe the color and arrangement of the dots. Can you see the individual dots without the aid of a magnifying glass? If so, how far from the screen must you be before you can no longer resolve adjacent dots?

The use of color dots to form images on television or computer screens is referred to as partitive mixing. Partitive mixing relies on the eye’s inability to resolve closely spaced objects. In the case of a TV screen, the objects are red, green, and blue light-emitting phosphors. Phosphors for each of the primary colors are clustered together in groups of three. In all, there are over 200,000 of these primary clusters. At a sufficient distance from a TV screen, the eye interprets each triad of colored dots as a single color.

Which phosphors are emitting light when a television screen or computer monitor is white? Black?

Color Mixing Demo

In Stock SKU: P2-9550
$39.00

Primary Color Light Sticks

In Stock SKU: P2-8100
$29.00

Station 11a: Subtractive Color Mixing

Key Concept:
Three colors of filters or pigments- often cyan, magenta, and yellow- can be absorption. This process is known as subtractive color mixing and is used in color printing and watercolor painting.

Setup Instructions:
Provide color filters that students can stack and look through. Part b requires printed material with cyan, magenta, yellow, and black test squares on the side. Color printed boxes from the grocery store often have these squares under the bottom flap.

Color Filters Set

In Stock SKU: 33-0190
$12.00

Station Instructions:

Hold a green and cyan filter together so that light from the lamp passes through both filters before entering the eye. Carefully observe the color(s) are visible through both filters. Record your observation below. Repeat this procedure with the combinations of colored filters listed below.

green + cyan = ?

green + yellow = ?

green + magenta =?

red + cyan =?

red + yellow =?

red + magenta =?

blue + cyan =?

blue + yellow =?

blue + magenta =?

red + green =?

red + blue = ?

green + blue =?

Your observations may now be used to deduce the transmission characteristics of cyan, yellow and magenta filters. In terms of red, blue, and green, list the colors that are transmitted by each of the following filters:

cyan:

yellow:

magenta:

cyan + yellow + magenta:

Cyan, yellow, and magenta are called the “subtractive primary colors.” A cyan filter may be thought of as a “minus red” filter because it absorbs red light.

What color does a yellow filter absorb?

A magenta filter?

Station 11b: Color Printing

Both colored filters and pigments selectively absorb certain colors. Cyan, yellow,
and magenta are referred to as the subtractive primaries because in the proper combinations they may be used to produce any color in the spectrum. For this reason, cyan, yellow, and magenta are used in painting and color printing.

Examine the flaps on the boxes provided. There you will see cyan, yellow, and magenta “test dots” indicating the colors used to print the box. These same colors are clearly visible on the color inkjet printing cartridge. Use a magnifying glass to view a color picture in a magazine or book or a colored image produced by a color inkjet printer.

What do you observe?

Ok the smorgasbords done…what now?

Many times students miss some important aspect of an activity. Therefore, it is often good for the teacher to repeat some of the activities done by the students as class demonstrations. In this way the teacher can focus student attention on the essential element of selected activities. Drawing on students to explain what they see and why they think it happens gets them actively involved in a communal learning process while allowing the teacher to assess understanding.

In addition to revisiting smorgasbord activities, the teacher can further amplify basic principles through the use of novel demonstrations. These demonstrations may be used to further clarify concepts and illustrate real-world applications of the basic principles being studied. Here are some extra resources for light & color activities:

The Art Institute of Chicago Science, Art and Technology
http://www.artic.edu/aic/students/sciarttech/

Optical Illusions and Perception
http://www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/Illusions.html#intro

Optical Society of America
http://www.optics4kids.org/

Teacher Demonstrations

Egg with soot dropped in the water goblet

Silver egg setup

The “Silver Egg” Demo

Use tongs to hold a normal egg in a candle flame until it is covered with soot. Drop the soot-covered egg into a glass of water. A considerable amount of the light traveling through the water is totally internally reflected when it encounters an air layer that adheres to the soot. Since most of the light is reflected, the egg appears to have a silvery, shiny surface.

The egg will appear silvery until the air dissolves into the water, which only takes a couple of minutes.

Look closely to observe what happens to the small fraction of light that passes through the air layer.

Color Subtraction Demo

In Station 11, students experimented with color subtraction. You can reinforce their observations with a more quantitative demonstration.

Print four separated color CMYK images. Print these images as separate transparencies. Download Color Subtraction Demo PDF.

Place the magenta, cyan, yellow, and black images of the Palace on top of each other so that the four images are perfectly aligned. Now place the combination on the stage of an overhead projector or light table. What do you see?

How does this combination of images produce a full-color image? (Hint: think subtractive color mixing.)

Light Wave Communication System

Using simple equipment, your students can transmit voice and music over a beam of light.  First attach a red LED to the earphone jack of a radio, tape or CD player.  This may be accomplished by using a cord with a mini-plug on one end and alligators clips at the other.  The signal from any of these devices will cause the LED to flicker. (Note: The LED will not light if the output of the electronic entertainment device is insufficient.  Placing a 9 V battery and a 470ohm resistor as shown should remedy the problem.)

The modulated light from the LED contains information relating to the frequency and intensity of the audio signal. A photocell or photodiode, connected to an amplifier/loudspeaker, is used to receive the modulated signal. The quality of the received signal is quite amazing!

Students enjoy seeing how far the signal may be transmitted. They should be encouraged to experiment with various optical devices such as lenses and optical fibers in their attempt to extend the range.

Exploring a Single Use Camera with Built-In Flash

Used single-use cameras can be obtained free or very inexpensively from photo developers. Single-use cameras contain a number of rather sophisticated components. Experiment 10.1 in Light Science describes an entire optics laboratory using single-use cameras. In this laboratory you will investigate the workings of a single-use camera and, in the process, become familiar with elements common to all cameras.

Download the print friendly pdf version! A Smorgasbord of Optical Phenomena.

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