# Light & Color

## You’re Getting Warmer! [W/Video]

The Little Shop of Physics has developed a series of videos called Flash Science, which show some exciting experiments that can be done with everyday items to demonstrate physics principles in a unique way. All of these experiments have been designed to be done by trained adults using proper safety equipment.

### Heat

In physics, heat is something you do; it’s a verb. It is defined as the thermal (non-mechanical) transfer of energy. When you heat an object, you transfer energy to it, which can raise its temperature or even cause a phase change. Traditionally, three sources of heat transfer are cited: convection, conduction and radiation.

Radiation is the transfer of thermal energy using electromangetic waves, which includes visible light, infrared radiation, ultraviolet radiation, x-rays and microwaves and radio waves. A camera flash is designed to give off a whole lot of visible light in a short amount of time. The black ink in the newspaper absorbs this radiation and increases in temperature, while the blank paper reflects the light and does not warm up nearly as much.

### Conduction and Convection

When a flame is held underneath a balloon, it’s no surprise that the balloon pops. The flame is at a high enough temperature to heat and melt (or even burn) the balloon, and the air under pressure inside quickly escapes. However, when the balloon is filled with water, the flame no longer pops it. The balloon is very thin, and the thermal energy quickly gets conducted to the water on the inside. The water has a very high heat capacity, so it takes a large amount of energy to increase the temperature of the water.

The water is also effective transferring the thermal energy away from the flame. The water will undergo convection; the warm water by the flame will move upwards, and be replaced by colder water coming in from the sides. Also, since water evaporates at 100°C, liquid water has a limit on how high of a temperature it can reach.

### Evaporation

Evaporation is an extremely important and sometimes overlooked form of thermal energy transfer. Evaporative cooling is the mechanism behind human sweating, and the energy stored in evaporated water is extremely important in the Earth’s weather system.

In this video the flame hounds are soaked in a mixture of rubbing alcohol and water. While the alcohol burns, and releases thermal energy, the water evaporates and takes much of that thermal energy away from the flame hounds, so that it does not burn!

If you’re careful, you can even hold flaming bubbles in your hands!

### Plasmas

Running electricity through the graphite pencil-lead causes the tip to get extremely hot, so hot that the graphite vaporizes and the vapor ionizes. These hot ions are used to cut aluminum foil, similar to how a plasma cutter or arc cutter works.

### Erasing With Heat

Some erasable pens use thermochromic ink, which changes colors from dark to light when it is heated. When the ink is cooled (such as through the evaporation of a liquid), the ink becomes dark again. With this ink, you can erase and re-write messages over and over again!

## 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).

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.

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.

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.

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.

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.

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.

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.

## Sunset Egg

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Radiation is the transfer of thermal energy using electromangetic waves, which includes visible light, infrared radiation, ultraviolet radiation, x-rays and microwaves and radio waves. A camera flash is designed to give off a whole lot of visible light in a short amount of time. The black ink in the newspaper absorbs this radiation and increases in temperature, while the blank paper reflects the light and does not warm up nearly as much.

Let us know in the comments below if you think this demo is something you would use in your classroom.

This video was produced by The Little Shop of Physics at Colorado State University in partnership with GE.

## DIY: Erasing With Heat [W/Video]

The thermochromic ink used in erasable pens has a unique quality; it changes color with temperature. The molecules in the ink are oriented in layers, and when light passes through, the wavelength with the greatest constructive interference is reflected back. When applying heat in excess of 140℉ to the ink, the color changes form dark (black) to light (clear – appears to disappear). The change in temperature changes the spacing between the layers of molecules in the ink which changes the reflected wavelength leading to different colors. When the ink is cooled (such as through the evaporation of a liquid), the ink becomes dark again. With this ink, you can erase and re-write messages over and over again! NOTE: Please pay attention to the safety concerns expressed in the video when using as a demo.

Let us know in the comments below if you think this demo is something you would use in your classroom.

This video was produced by The Little Shop of Physics at Colorado State University in partnership with GE.

## Newton’s Eye Poke Experiment [W/Video]

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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## Fun and engaging activities using the Energy Stick [W/Video]

Welcome to our March 2015 Issue of our CoolStuff Newsletter. This month, we are featuring a simple, safe and Cool device called an Energy Stick. Physics teacher James Lincoln demonstrates several experiments that help students understand the principles of electric current and light. James has authored many of our past CoolStuff Newsletters, and teachers have really enjoyed his insight, passion and creativity. We encourage you to let us know what you think, and please feel free to contribute to the conversation by submitting a comment. Thank you for being a CoolStuff subscriber – enjoy!

Arbor Scientific
We find the CoolStuff

The Energy Stick is a fun and easy way to demonstrate many of the principles of electric current and light. These topics are important for both the physics and the chemistry teacher. In this article I will outline several of these such experiments including new ones not seen anywhere else.

1) GETTING STARTED
To operate the Energy Stick, make bodily contact with both ends of it. This sends a microcurrent through your body which is amplified by the circuit inside and sent to the LEDs and speakers inside. This is how you can know whether a measurable electric current is able to flow from one side of the stick to the other.

2) CONDUCTIVITY OF VAROUS OBJECTS
One of the first experiments you will want to do with the Energy Stick is check what other objects conduct electricity. This is a good lesson in the properties of metals for chemistry, physics, or middle school science. You will find that mostly metals conduct electricity. I have also found that even distilled water conducts electricity well enough to have an effect. This should not be a surprise since the human body is mostly water and the human body works well.

Miscellaneous household items are good candidates for conductivity tests.

The open circuit fails to light

Closing hands completes the loop and current can flow

3)THE IDEA OF A COMPLETE CIRCUIT
An important lesson is that for current to flow the circuit must complete a closed loop. Thus, if there is a break anywhere in the circuit electricity cannot move through any part. This can be dramatically demonstrated by having several members of the class join hands in a ring and complete a very large circuit.

The Energy Stick’s Voltage is only about 30 milliVolts. The current output depends on the circuit it is connected through but is always only a few milliamps at most.

4) INVESTIGATIONS OF THE CURRENT
Connecting the two ends of the Energy Stick with a wire activates the circuitry inside. You can connect that wire to other electric devices such as a ammeter and voltmeter. In both cases the measurements will be quite small so it helps to have sensitive meters. The Energy Stick is a safe way to familiarize students with these probes.

5) THE PLASMA GLOBE and the Frequency of Light
A plasma globe can also be used to turn on the circuitry of the Energy Stick. Since the circuit inside amplifies very small currents, the electric field near the plasma is enough to get an effect. Inside the Energy Stick the red, green, and blue diodes turn on at different distances. This is a lesson in modern physics and chemistry. That is the meaning of the formula E=hf.

The Red Diode is the first to turn on.

As the Energy Stick is brought nearer the plasma globe, the other colors turn on. Next green, then blue last.

Red light having a lower frequency (longer wavelength) than blue and green light will can be produced at a lower voltage (energy/electric charge). Therefore, the blue diode is the last one turn on. This recalls the idea of the photoelectric effect that it is not the brightness of the light but its frequency that determines how energetic it is.

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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## 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.

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.

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

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 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: 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 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 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.

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

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).

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!

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

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.

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.

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

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.

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

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).

Above: UV Light is turned off.

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.

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).

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.

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.

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 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.

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.

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

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 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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## A Visit to Isaac Newton’s Home [W/Video]

When I visited England this summer, I had the opportunity to walk in Isaac Newton’s footsteps at his birthplace, Woolsthorpe Manor.

It is a little tourist museum far from the major train stops. You will probably have to take a long taxi ride from the train station at Grantham, but it’s not so far from London that it’s more than a day trip.  Woolsthorpe means wool farm, and it still is a wool farm where you can find long-haired Lincoln Sheep grazing in the meadows.

It is everything you would imagine a farm in the idyllic English countryside to be, with lightly rolling green hills, a stone cottage, and barns. The home and birthplace of Isaac Newton still stands as it did during his lifetime: a proud stone cottage with many adjacent stables and farm houses. Today, every part that is not empty is a museum.

During the plague years, 1665-1666, Newton fled his college at Cambridge and retired home to his mother’s stone cottage beside a grove of apple trees.  He was a self-taught student of mathematics, and by the time he returned, he was the top mathematician in Europe.

It was in the apple orchard here in Woolsthorpe that Newton developed his Three Laws of Motion and Law of Universal Gravitation. The apple tree that inspired the latter is supposedly still growing here. It is gnarled with age but still gives apples.

The markers tell the story of how the tree that inspired the young Newton’s gravity theory fell victim to gravity itself during a major storm.

At well over 400 years old, this tree miraculously still bears apples and inspires those who make the pilgrimage. As the sunlight broke through the clouds and the birds sang far from the city sounds it was easy to imagine what this place was like in Newton’s time and how one could think clearly here.

The peace stretches far across the land, and there is a contemplative feeling in the air; it seems like a place where there is nothing much to do except think of ideas.

Newton was born here on Christmas Day 1642, and grew up here learning to build little mechanical toys.

Young Newton often neglected his farm duties, such as mending fences and tending to the sheep, in favor of reading and experimenting. His father passed away before he was born, and his mother remarried and left him in the care of his grandmother. These stories and others can be learned while visiting the museum that was his home.

The main building itself has five rooms, and these are decorated in the style of the era, with signs and exhibits throughout. There is even a little hands-on science museum where visitors can try out some of the classic experiments associated with Isaac Newton.

Also on site, you will find a human sundial, a place for a picnic, and the famous experiments on optics that were Newton’s other great contribution to physics. There are prisms, reflecting telescopes, and light color mixers.

When Newton came of age, he left the countryside to attend Trinity College in Cambridge. Here, you can find cloned relatives of his apple tree, grafted, through vegetative propagation. One of the trees is in the Cambridge University Botanic Garden.

Cambridge is a great day trip too, and Trinity College is beautiful, thanks to King Henry the VIII’s money. Near the front gates you will find another apple tree, right outside Newton’s office, where he lived and worked as a research professor for almost 30 years.

So where is Sir Isaac Newton now?

Some of his books and locks of his hair can be found at Cambridge and Woolsthorpe. Even a copy of his death mask resides at Woolsthorpe. The memorial and grave of Sir Isaac Newton is at Westminster Abbey in London. This is not far from where he was president of the Royal Society of London. Which also has markers that honor their famous former president.

Of all the sights, none can equal the quiet, contemplative air of the orchard in front of the stone cottage at Woolsthorpe Manor. As a physicist, this is truly a place for spiritual fulfillment.

It was during the plague years, that Newton escaped here and was inspired to write his Laws of Motion and Theory of Universal Gravitation. Spend an afternoon here and you too, may have a great idea.

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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## Top 10 Demonstrations with Tuning Forks [W/Video]

I have been using tuning forks in my classroom for 10 years, and in each of those years I have discovered several new tricks.  I hope you can learn many of these from this publication.  For a more complete treatment and my references, please see my article in “The Physics Teacher” March, 2013.

0.     General Usage

When a tuning fork is struck it will vibrate wildly in unintended ways.  Imagine putting your arms straight above your head and clapping.  That is the proper motion of a tuning fork.  The problem is that it is also wobbling at the “elbows.”  You can get rid of the unwanted vibrations by touching gently near the joint after striking the fork.  The vibrating tuning fork should be almost silent when used properly.  Hold the tines near your ear and you will hear it clearly.  It is best to hit the tuning fork on a knee or the ball of your hand, avoiding metal on metal.  This is because when tuning forks become chipped they change their inertia and will vibrate at different frequencies.  Spin the fork as you listen and notice that it is loudest right between the tines.  (Constructive Interference.)

1.      Water Dip

Putting a tuning fork in water is one of the best ways to get students accustomed to handling it.  Give a tuning fork to each student or every other student.  Set out several cups of water.  It is always a surprise to see the splash, students will gasp.  These introductory activities are important for laboratory management because the tuning fork is a fun toy and does require some getting used to; the sensation of hearing the tines vibrating is new and somewhat alarming.  It is also a good idea to have boxes or desks or the whiteboard cleared off for students to place the base of the tuning fork against and cause the vibration to resonate.

2.     Strobe Lights

A fun demonstration is to put the tuning fork in front of an adjustable strobe light (or a CRT computer monitor).  The strobe light can be adjusted to make the vibration appear slower or even stop!  This works better on a larger tuning fork.  The flashing must match the frequency of the tines, or be very close.  I found my 100 Hz tuning fork to be 99 Hz after investigation.

This effect comes from the strobe light “animating” the fork slowly through time by only making it visible after almost full cycle has passed.  At that moment, the fork will look as though it has only moved slightly.  The difference between the strobe rate and the tuning fork frequency determines the perceived rate of vibration.  The CRT monitor can also act somewhat like a strobe light, but because of its trace across the screen, it causes a wobbly effects in the vibration.

3.      Oscilloscope

Verifying the frequency of the tuning fork can easily be achieved by using an oscilloscope.  This is done by hooking a speaker removed from its housing to the scope’s leads.  You will need to have a proper connection (usually a BNC connector with probe) to achieve this.  You can also use a microphone.  Hold the tuning fork up to the speaker and adjust the settings.  You can see that the fork’s tone is a pure sine curve.  Compare this with the human voice or other instruments such as flutes and kazoos.  Also, try comparing tuning forks of various frequencies and noting the different periods & wavelengths.

4.       Resonance

Two tuning forks that are the same frequency can be made to resonate audibly if the vibration is loud enough.  For this purpose, I prefer using the large box-mounted versions.  Most large glass or wooden objects will have so many resonance frequencies that any tuning fork will cause them to resonate.  Tuning forks that are not the same frequency will not resonate.   The important phrase to understand is “Forced vibration at natural frequency causes resonance.”  Where “resonance” is high amplitude oscillation.  We all experience resonance when singing in the shower; the longer notes resonate better and it makes our voice sound purer in tone.  Also, when our wheels are not aligned in the car and we drive at the natural frequency of our shock springs the car will resonate up and down – but only at specific velocities.

5.      Sound via Light

Shine a laser on a solar cell from across the room, hook that solar cell to a set of computer speakers and demonstrate the transmission of sound via electromagnetic waves.  This is analogous to radio signals that we listen to because they are also modulated electromagnetic waves.  The laser’s color doesn’t matter much.  I sometimes add smoke to enhance the demo visually.  You will get a less distorted sound if the fork is further into the beam rather than just barely touching it when vibrating.  Clipping to the speakers may require some trial and error.  The “male end” of a stereo cable has its tip going to the left speaker, the middle ring goes to right, and the inner metal goes to ground.  Clip one end of the solar cell to either left or right, but you must clip the other to ground.  A guitar amp will work fine, probably even better.  Clip similarly to the plug of the guitar cord.

6.     Interference

Demonstrating interference is important because it is a property of all waves.  In this case I am using two close frequency waves to show the phenomena called “beats.”  Beats are sometimes also used to tune musical instruments (see #10).  The beating frequency is the difference between the interfering frequencies, the note you hear is the average of the two original frequencies.

This pattern can also be achieved by taking two identical tuning forks and heating one of them with a fire.  (I demonstrate this in the introduction to the video.)  Be sure to wear a hot glove!  The heat reduces the Young’s modulus (similar to spring constant) of the aluminum and the vibrations no longer match.  You can easily tell the difference even with a non-musical ear.

7.     Measure the Speed of Sound

With a tube and some water in a bowl it is easy to measure the speed of sound by resonating it with a tuning fork.  The wavelength of the sound must match the length of the tube, but the whole wave doesn’t have to fit inside for this to happen.  Most commonly, the bottom is sealed and becomes a node (a place where the air can’t move) but the top is open and the air can vibrate liberally (anti-node).  The smallest fraction of a standing wave that can fit in here is a ¼ wavelength.  Multiplying wavelength and frequency gives the velocity of sound, usually within 1% error!  If you don’t have a glass tube, this demonstration can also be done with a graduated cylinder that is being filled with water until resonance is achieved.

8. Smoke and Mirrors

Reflecting light from the end of a mirrored tuning fork can lead to exciting effects.  It gives us a chance to view the motion of the fork by amplifying it as the reflected light is projected across the room.  In the video, I add smoke to help you see the beam.  Because the tuning fork’s motion is sinusoidal in time, it can be made to trace a nearly perfect sine curve in space when it is rotated smoothly at a point far away.

Lissajous Figures are an old method by which tuning forks were tuned.  Excess fork was shaved off to bring the frequency down.  These days, Lissajous Figures are mostly they are used to analyze electromagnetic oscillations in LRC circuits, but originally they were produced by tuning forks reflecting light that is pointed at two mirror loaded forks vibrating at 90 degree angles.  When the frequencies are in ratio you get a Lissajous Figure.  They come in the shape of donuts, pretzels, fish, and other edible items.  It is best to have the forks close, but the wall far away because that will increase the size of the figures and reduces aiming difficulties.

9. Strike a Chord

Tuning forks come in various frequencies.  You can use them to inform students that music is a branch of physics.   With help you can create chords or even play songs with your students.  Take time to notice that there are specific ratios between notes that are in harmony.  For example, between G and C there is a 3/2 ratio – this is called a fifth.   Between E and C is a 5/4 ratio – this ratio is used in the C major chord.  And between C and A is a 6/5 ratio which is used in the A minor chord.  All octaves (such as middle C and the next C above middle C) are separated by a doubling of the frequency.  These ratios apply to both scientific and musical tuning fork frequencies and it is a fun game to try to discover them by reading your tuning fork labels.

10. Tuning

Tuning an instrument with a tuning fork can be done in many ways.  Typically, the tuning fork is merely listened to or held to the body of the instrument while it is tuned by ear.  But the fork can also be used to resonate the strings into vibration (if they are already in tune).  A completely different method is to strike the note and listen for beats as the sound from the instrument interferes with the sound from the tuning fork.  As the two are brought into tune, they will beat less and less frequently until they are matched with no beating.

It is important to note that the scientific tuning forks do not match the musical frequencies.  For example, A 440 Hz is a musical note, whereas A 426.7 Hz is the scientific note.  In the figure, my guitar tuner thinks my scientific tuning fork is flat by a half step.  The scientific scale is arranged around middle C being 256 Hz (C is 261.6 Hz on the musical scale).  The setting of the musical scale was done somewhat arbitrarily done by German musicians in the early 20th century.  The scientific scale is convenient where all C notes are a multiple of 2; for example, the first C above middle C is 29=512 Hz.  Many of the other frequencies are also whole numbers, such as G 384 Hz and D 288 Hz.

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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## Color Me Excited, Seeing spectra in a whole new light [W/Video]

Teaching the spectra of visible light can be an engaging classroom activity. But, it’s always been challenging to find ways to go beyond simple passive demonstrations.

Hand-held diffraction “rainbow” foil (sometimes mounted in cardboard glasses) can be fun. But students often have difficulty even spotting the spectrum. “I can’t see it!” is the common complaint. And because each student observes their own “private” spectrum, it can be difficult to draw them into a discussion about what they’re seeing.

An exciting new way to augment, or even replace, rainbow foil activities is to display a live spectrum on a computer overhead projector. When you use the RSpec-Explorer video spectrometer system, everyone observes the same thing. Having a concrete example in front of the classroom makes teaching the material much easier. And you’ll find that a live video captures the students’ attention far more effectively than any other teaching aid we’ve used for this topic.

LED Array Spectra
Many educators prefer to begin by showing the spectra of individual colored LEDs, The LED colors are pretty – and, yes, that helps! (See accompanying video.) By starting out with familiar LEDs, we help the students connect what they’re seeing to their everyday experiences.

Using an LED array allows us to display multiple spectra simultaneously, clearly demonstrating that each individual wavelength is diffracted by a different amount. (See Figure 1) This novel presentation often has students leaning in as they contemplate something so attractive and so different from what they’ve seen before. There’s a lot of opportunity here for you to challenge your students to explain what they’re seeing.

The white LED at the top of the column (visible in Figure 1 and video) helps students see how the individual stacked colors are in a rainbow.

As the video that accompanies this article shows, we can easily transition from qualitative to quantitative observations using the graphing capability in the RSpec-Explorer system. When students see the intensity graph of each individual color, it helps them see how one moves from raw observations to scientific data.

We’ve found that the rapid changes of the live intensity graph as we interact with our light source transforms what was previously a monotonous visual experience into something that really grabs students’ attention.

Gas Tube Spectra
Traditional gas tubes are an easy next step after the LED array. Once again, a live video display rather than idiosyncratic hand-held “rainbow” foil assures that our students all see the phenomenon being discussed. The live intensity graph alongside the original spectrum is very effective. Students can easily see how the colorful qualitative raw data becomes much more meaningful when graphed. Figure 2 shows the real-time spectrum of a Hydrogen gas tube (red). The blue vertical lines are shown by the software to indicate where we would expect to find Hydrogen Balmer peaks. You can see the red data peaks match up with the blue.

We like to click a tool bar button in the software to freeze the Hydrogen spectrum graph. Then we swap the tube out for a Helium tube. Seeing the spectra of both gas tubes on the same x-y axis (Figure 3) helps make it clear that a spectrum is a chemical fingerprint, differing from element to element.

The video that accompanies this article shows how we can use a reference library to readily identify the contents of a “mystery gas tube.” This is a great demonstration for your class. Or, you can challenge your students in a hands-on lab to determine the contents of an unidentified gas tube. Of course, using spectroscopy to identify unknown objects is a cornerstone of astronomical research. You might want to tie this activity into an astronomical discussion or about the Curiosity rover that recently landed on Mars – both also use a the same “fingerprint” matching to identify “mystery” objects.
http://msl-scicorner.jpl.nasa.gov/Instruments/ChemCam/
http://mars.jpl.nasa.gov/msl/mission/instruments/spectrometers/chemcam/

When is yellow not yellow?
The availability of a live video spectrum opens up the possibility of all sorts of labs on light and color. For example, the video that accompanies this article shows two yellow spectra: one of a lemon, and one of a yellow cell phone screen. The two “yellows” have very different spectra. The cell phone screen spectrum contains no yellow at all – just green and red. This is a wonderful illustration of how an RGB monitor can “trick” the human eye into seeing yellow.

Burning to do Flame Spectroscopy
As the accompanying video shows, you can also observe spectra of some common elements by burning them in a Bunsen burner. Using flame salts intended for this purpose, different elements will exhibit different emission lines. This activity is a bit more challenging than gas tube spectroscopy, but can be instructional and rewarding for your students. Seeing the spectrum in real-time makes the process easier. Students can record the live video. And they can capture bitmap/jpg images of the graph for inclusion in their lab reports. Students can also export XY graph points of the intensity graph to a text file for additional analysis at a later time.

Astronomical Spectroscopy
The software that comes with the RSpec-Explorer system has advanced features that allow you load astronomical spectra. You can explore on-line Hubble data. Or, open the Mars rover ChemCam data in the system and study their spectra. Although this is a somewhat more advanced activity, it’s a natural one to follow up with after your students understand gas tube spectra.

Conclusion
A real-time video spectrometer makes it much easier to teach light and spectra. Your entire class can share the same real-time view of your gas tubes and other light sources. And, at the same time, they can see the resulting intensity graph. Students won’t have to struggle finding a spectrum in a tiny piece of plastic. And you’ll find it’s liberating to know everyone can see the examples you’re discussing. Walk up the screen or use a laser pointer to call out important points. Capture screens and include them in classroom handouts. Or, have your students operate the system in “identify the mystery tube” labs.

Tom Field
Field Tested Systems, LLC
Seattle, WA USA

Tom Field is the founder of Field Tested Systems, and is also a Contributing Editor at Sky & Telescope Magazine. He has been involved in the education field for the past two decades. With a passion for science education, Tom is has been on teams large and small that have developed hardware and software applications in use today by thousands of users on all seven continents.