# Measurement

## 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!

# Back to School Means Back to STEM

The Next Generation Science Standards (http://www.nextgenscience.org/next-generation-science-standards) identifies eight practices of science and engineering as essential for all students to learn. These are:

1. Asking questions (for science) and defining problems (for engineering)
2. Developing and using models
3. Planning and carrying out investigations
4. Analyzing and interpreting data
5. Using mathematics and computational thinking
6. Constructing explanations (for science) and designing solutions (for engineering)
7. Engaging in argument from evidence
8. Obtaining, evaluating, and communicating information

One of the best ways to implement all of these practices is through the growing practice of Project Based Learning (PBL), defined by Edutopia as “a dynamic classroom approach in which students actively explore real-world problems and challenges and acquire a deeper knowledge” (http://www.edutopia.org/project-based-learning). The duration of these projects can be as short as a single class period or last throughout an entire school year, but typically last from 1-3 weeks.

This past summer, Grade 6 – 9 science teachers, math teachers, and administrators from Warsaw (IN) Community Schools partnered with science and math educators from Ball State University during a two week summer institute to design classroom projects, strengthen science and math content knowledge, and refine inquiry practices. The Arbor Scientific “Pull-Back Car” (http://www.arborsci.com/pull-back-car) was featured in a “mini” PBL activity to illustrate how science, technology, engineering, and mathematics (STEM) can be integrated into an authentic data collection and analysis activity.

Participants were divided into 12 groups and each group of 4-5 participants was given a pull-back car. Each participant played the role of a quality control engineer, whose job description may include taking “part in the design and evaluation of the product” and being “responsible for making sure that the materials meet the requisite standards and that the equipment works correctly” (http://www.wisegeek.com/what-is-a-quality-control-engineer.htm). Each group served as a team of quality control engineers who were instructed to design and conduct tests on these cars to determine if Arbor Scientific should continue selling them.

The engineering teams addressed four questions about the cars:

1. 1. How consistent is the distance an individual pull-back car travels after being pulled back a specified distance?
2. 2. How consistent is the amount of time an individual pull-back car travels after being pulled back a specified distance?
3. 3. How straight does an individual pull-back car travel?
4. 4. Do all pull-back cars behave similarly?

Each team was charged with designing an experiment to answer their quality question, conducting the trials, analyzing their results, summarizing their findings on poster paper, and reporting their results to the large group.   Groups scattered in and around the building to design and conduct their trials.

After completing the data collection portion of the activity, groups returned to their tables to analyze their data and report their results.

After each group had completed their tasks, each group presented their findings. Examples of summary posters are shown below.

Once the participants had a good idea of how consistent the cars were, these cars were again used later in the summer institute to investigate additional inquiry questions, such as:

1. 1. How does the distance the car is pulled back before release affect the total distance it travels?
2. 2. How does the distance the car is pulled back before release affect the total time it travels?
3. 3. How does additional weight affect the distance the car travels when pulled back a specified distance?
4. 4. How does additional weight affect the total time the car travels when pulled back a specified distance?
5. 5. How does the angle of incline affect the distance the car travels when pulled back a specified distance?
6. 6. How does the angle of incline affect the total time the car travels when pulled back a specified distance?

Despite slight inconsistencies in the behaviors of the cars, participants in this summer institute agreed that the Arbor Scientific Pull-Back Car is an excellent inexpensive product that can be used in many different investigations – especially when one wants to integrate aspects of STEM. These cars can be used at any grade level and provide countless opportunities for students to engage in authentic scientific inquiry. They also provide an inexpensive way for math teachers to incorporate real world data collection and analysis. As a science educator, I heartily recommend this product and can honestly say that I consider it the ultimate inquiry device!

Dr. Joel Bryan

Ball State University

Muncie, Indiana

Dr. Bryan teaches at Ball State for the Department of Physics and Astronomy. He taught all levels of high school physics (Pre-AP, AP, conceptual) and a variety of mathematics courses for 13 years before receiving his Ph.D. in curriculum and instruction at Texas A&M University.

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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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## 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).

Here are up-close pictures of Mentos candies.

Can the microscope unlock their secret powers of soda exploding?

The Mint Mentos display a pitted surface.

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.

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

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

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

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.

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.

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.

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

To hold the world in a grain of sand.

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.

A wasp from my students’ collections.

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 ).

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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## Kinetic energy into thermal energy [W/Video]

Watch how silly putty and simple steel spheres can be used to demonstrate the conversion of kinetic energy into thermal energy! (And you thought Silly Putty was for copying comics from the newspaper!)

One of the most important concepts in physics is the principle of energy conservation, especially the idea that energy can change forms and be transferred from one object to another. An important application of this idea is that kinetic energy can be transferred to heat, which is just another form of energy. One way to demonstrate this idea is to have students simply rub their hands together, showing that friction transfers energy into heat. However, it may be less obvious to students that impacts also generate heat, since the role of friction may not be as clear when the objects are not actively rubbing together. One easy way to demonstrate this is with a set of Colliding Steel Spheres. When I use this demo in class I ask for a “brave student volunteer.” I have the student volunteer hold a sheet of paper vertically and then I dramatically smash the spheres together with the paper in between. A small hole results and I have the student examine the hole, in particular noticing that there is a smell of burning paper. This clearly shows the class that the kinetic energy of the moving spheres is changed into heat energy when the spheres collide. I then typically make several more collisions to drive home the point, and sometimes allow students to try it for themselves.

Another dramatic and visual way to demonstrate this concept uses silly putty, a hammer, and a temperature probe connected to computer data acquisition system. Simply embed the temperature probe inside the silly putty, start the data acquisition, let the temperature come to equilibrium and then smash the hammer down on the silly putty. You should then see a rise in temperature recorded on the screen, again showing how kinetic energy is transferred into heat energy, resulting in a rise in temperature of the silly putty.

Another great way to show energy conservation, and connect to thermodynamics concepts, is the Fire Syringe. This is basically a piston that you can use to rapidly compress a column of air with a bit of cotton ball; the rapid compression heats the air and ignites the cotton ball. This again is an illustration of converting kinetic energy into heat; you can also present this in the context of the idea gas law, where the compression increases the temperature. A couple of things to watch out for when using this device: the seal needs to be good, so the o-rings should be lightly lubricated and when you compress the piston it should bounce back rapidly; use a small bit of cotton ball and pull it apart so there is a lot of surface area to heat. You need to give the piston a quick compression, and it helps if the cotton fibers are near, but not sitting on, the bottom of the column.

These demos are visual and, for many students, surprising, and can really drive home the idea that energy can change forms and particularly that kinetic energy can be changed into heat energy.

Brian Thomas is an Associate Professor of Physics & Astronomy at Washburn University’s College of Arts & Sciences His Career Accomplishments include but are not limited to: Principle Investigator on a \$500,000 research project funded by NASA’s Astrobiology program, in collaboration with the Smithsonian Environmental Research Center, author or co-author on approximately 20 peer-reviewed articles. Invited speaker for several international conferences. His work has been featured in news articles, as well as in several television documentaries.
washburn.edu/our-faculty/brian-thomas

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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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## Playing in Galileo’s Lab (part 1)

As I was watching a kayak quietly slip under the Ponte Vecchio in the morning light, I was thinking what it must have been like for Galileo Galilei while living and teaching in Florence, Italy, looking down every day at the beautiful Arno River. In Galileo’s time, the Ponte Vecchio bridge, the ONLY bridge in Florence to survive the bombings of WWII, was populated by butcher shops with meat cutters throwing the leftovers into the Arno River.

Ponte Vecchio bridge over the Arno River

Sculpture of Galileo

While the other members of my family shopped on the famous Ponte Vecchio bridge, now replaced with gold and diamond jewelry shops, I was first in line to see the gold standard in science museums, Museo Galileo or the Galileo Museum.

It was 8:45 AM in Florence, and I was waiting for one of the highlights of my 2012 Italian family vacation, which was the chance to see the resting place of original creations from one of the greatest minds of the 16th century.  Like a little kid waiting in line for the newest roller coaster at Disney World, I stood by the door with eager anticipation, excited to finally see in person what I had only seen in college textbooks or on YouTube. As the door opened at 9:30 AM, I paid my 9 Euros and raced up the stairs with great anticipation.

When entering the Galilean room, you are met by three glass-enclosed bell jars that house probably the oddest, yet eeriest of all the displays of the museum, Galileo himself!  Because of Galileo’s status as a revered scientist and statesman, grave robbers and souvenir seekers wanted remembrances of the famous scientist and raided his remains.  Preserved under glass, in what could only resemble a saint’s reliquary, are Galileo’s vertebrae, molar, thumb, and middle finger. An ironic gesture to the world that he was right all along about the motion of the Earth and its place in the solar system. This item exemplifies the celebration of Galileo as a hero and martyr of science.

Displayed on the first wall was one of the most famous of all of Galileo’s experiments, the Inclined Plane. This apparatus used five small bells, along with a pendulum to provide an experimental demonstration of the Galilean law of falling bodies. The Law was demonstrated with the pendulum connected to the inclined plane, acting as a “timepiece”. The experiment consisted of releasing a ball from the top of the plane at the same time as the pendulum was swung. For each complete period of the pendulum, the ball would strike one of the small bells placed along the inclined plane at increasing distances, specifically arranged in the order of odd number distances. The experiment made it possible to measure the increase in the distances traveled by the ball as it rolled through equal time intervals starting from the rest position. The ringing bells would also provide an additional auditory observation of the ball’s constant acceleration during its fall.

Galileo’s Inclined Plane

In the background of the Inclined Plane was the Brachistochronous, an apparatus demonstrating the observable effects of a physical principle discovered by Galileo on November 29, 1602. Using geometrical methods, Galileo proved that a body takes less time to fall along the arc of a circumference than along the corresponding chord, even though the latter is a shorter path. The device consisted of a wooden frame with a cycloidal channel and a straight channel which was adjustable by means of pegs fixed in holes with brass rings under the cycloid.  By dropping two balls simultaneously down the two channels, he was able to observe that the ball falling down the circular channel reaches the bottom well before the ball traveling down the inclined plane.

Close-up view of Galileo’s Inclined Plane Bells

On the far wall was the original telescope made by Galileo consisting of a main tube with separate housings at either end for the objective and the eyepiece. The tube was formed by strips of wood joined together and covered with red leather and gold trim. The plano-convex objective, with the convex side facing outward, had a diameter of 37 mm with a focal length of 0.980 meters and magnification of 21X. However, the original eyepiece was lost. In 1611, Prince Federico Cesi, founder of the Accademia dei Lincei, suggested calling this instrument telescopio [from the Greek tēle (“far”) and scopeo (“I see”)].  Galileo designed ingenious accessories for the telescope’s various applications. Among those were the micrometer, an indispensable device for measuring distances between Jupiter and its moons, and the helioscope, which made it possible to observe sunspots through the telescope without risking eye damage.

Galileo’s Original Telescope

Galileo’s Original Projectile Motion Diagrams

On the far wall of the Galilean chamber was a gorgeous wooden instrument used for studying Horizontal Projectile Motion.  A ball would be rolled from the top of the cycloidal ramp, exit the ramp and pass through a series of metal rings placed in a parabolic pattern, all driven by inertia and gravity.  It is well-known that Galileo studied projectile motion and its effect on the motion of an object on Earth. His writings are well known and illustrate his fascination with the motion of falling bodies.

Galileo’s Projectile Motion Demonstration

What is amazing is that students STILL study projectile motion in the same way as Galileo did some 400 years ago.  Although not as artistically designed as Galileo’s version, the modern-day aluminum Horizontal Projectile Ramp still demonstrates the laws that govern horizontal projectile motion.

Museo Galileo showcased Galileo’s accomplishments throughout his life; his writings, his works, original devices and famous experiments, which allowed many scientists to build their theories upon.

The museum’s remaining rooms exhibited numerous other scientists’ work and laboratory equipment.  What I found absolutely fascinating was that the demonstrations used in today’s classrooms bear a strikingly similar appearance to what our predecessors used in their labs.  The more equipment I saw, the more I said to myself “Wait, I use that in my classroom!” or “That looks just like the lab I did with my students last year!”.  I realized that I was REALLY playing in Galileo’s Lab!  Given the equipment in the museum and the writings of the various scientists, I could have brought my class to Florence and had no problem teaching Physics in the SAME way I always had.  One device after another would demonstrate a Physics concept in the same manner that I might have done it.  Then I knew why, the very devices we “modern-day” Physics teachers are showing to our students, actually thinking that these are unique demonstrations that few have seen before, except in a catalog, are really 400 years old!  The following are many of the devices that I saw that day at the museum and after taking pictures of them, I tried to think of a cool way to prove my earlier point:  that our modern day Physics demos and device are frighteningly similar to their 400 year-old counterparts. Here are a few I encountered…

A Chemistry Lab Bench.I really would rather do my experiments at the 400 year old bench… Wouldn’t you?

Chemistry Laboratory Experimental Bench (circa 1600’s)

Set of armed lodestones used by Galileo for his studies on magnets (1600-1609).  These DEFINITELY look “armed”!

Galileo’s Armed Lodestones

Force vector table hasn’t changed much in 250 years!

Force Vector Table

That concludes part 1 of Playing in Galileo’s Lab. More to come including Galileo’s Inertial Mass Balance with Chair and Air pressure Demonstration with a balloon.

### About the Author

Buzz Putnam, Physics Teacher, Whitesboro, New York
Buzz is a 25 year veteran Physics and Nanotechnology teacher who has served as Whitesboro High School’s Science Department Chair since 1997. Buzz also conducts Teaching Methods classes for science teachers at Utica College of Syracuse University. In addition, he is part of the Cornell University Laboratory Development Team and a member of the Cornell NanoScale Institute for Physics Teachers. He is also a frequent presenter at NY, NJ, Texas and National Science Teachers Association Conferences and has won numerous teaching awards throughout his career.

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## Extending Our Senses: Indirect Measurement

Everything we experience comes to us through our five senses—sight, hearing, touch, smell and taste. While our senses are truly amazing, most of what goes on around us occurs unnoticed. Since we can only see a small range of the electromagnetic spectrum as visible light,  we can be in the vicinity of a radio-transmitting tower radiating 50,000 watts of power and be totally unaware of its presence. The fluttering of a hummingbird wing and changes in mountain ranges are undetectable to the average human. Extreme distances, both short and long, are equally elusive. We can see the dot above an “i”, but cannot see a grain of pollen. At the other outer limits of length, we can only imagine what a light year is.

Scientific research includes the study of subatomic particles as well as the mind-boggling distances that exist between the earth and neighboring stars and nebulae. This great breadth of investigation involves extending our senses and developing new ways of “seeing”.

Scientific instruments that enable us to overcome our sensory limitations have been, and continue to be, essential to the progress of science. The microscope and the telescope provide mankind with windows to two previously unseen worlds. The stroboscope has enabled us to “freeze” motion. X-rays have provided a non-invasive way of probing the body. Radio telescopes enable us to extend our grasp to the far reaches of space. Cloud and bubble chambers allow us to study events occurring on the subatomic scale.

In a similar way, the following experiments will allow your students to extend their senses and make measurements they never dreamed possible. They will determine the size of a molecule, time events that occur in an instant, and measure dimensions on an astronomical scale. In the process, they will learn how scientists make observations and measurements in the invisible world.

Student Activities

### 1. Measuring New Heights

Key Concept:
Students are asked to indirectly measure the height of an object much larger than their available measuring instrument.

Teacher Instructions:

Give the students instructions below, and turn them loose! Give them time to plan in the classroom before going out as a group to make measurements. Give as little advice as possible. Their methods (and the results) may vary a lot, and that’s okay!

Student Instructions
The challenge: Determine the height of a tall object on the school grounds such as a flagpole, a chimney, or other structure identified by your teacher. You will only be allowed the use of a meter stick. You may think that you don’t have the knowledge to make such a measurement with so little equipment, but you would be wrong!

Before leaving the classroom, do some brainstorming with members of your group. You will be surprised to learn that you have the ability to measure such a tall object indirectly. Each group will be asked to share not only their value for the height of the object, but more importantly, their method. So be ready!

### 2. Blast Off!

Key Concept

The measurement method known as triangulation can be used to indirectly determine the heights of tall structures or the altitudes of projectiles.

Teacher Instructions

Demonstrate the calculations for the class before assigning each group a height or altitude to measure. Trigonometry is involved, but students really only need to do some basic algebra to grasp this concept. You can use triangulation to find the height of buildings or as part of other labs and activities like Bottle Rockets.

Student Instructions
Trigonometry provides an easy way to determine the heights of structures or even the altitude of a toy rocket. Trigonometry deals with ratios of the lengths of pairs of the sides of a right triangle. You may have heard of the sine, cosine, and tangent. Scary sounding? Perhaps, but don’t worry, they’re all just ratios. To make things easier, we’ll only consider the tangent.

The tangent of an angle (Θ, “theta”) is the ratio of the length of the side opposite the angle to the length of the side adjacent to the angle. In other words, it’s the ratio of side a to side b. This ratio increases as the angle of inclination Θ increases. The tangent for angles between 0 and 90 degrees may found in a table or calculator.

Suppose you fire a rocket into the air and wish to know its altitude. If you know the distance from you to the launch point (b) and the angle of inclination (Θ), you can find the rocket’s altitude (a) becausetangent Θ = a/b
or
a = b tangent Θ(Hint: You can look up the tangent of any angle from 0 to 90 in a table or by using your scientific calculator.)Voila! The tangent makes the indirect measurement of heights a snap.

fig.2

To actually carry out a measurement of a rocket’s altitude, you will need a protractor (an instrument used for measuring angles), a string with a small weight on the end (also known as a plum bob), a meter stick, and a tangent table or calculator.

After tying a weight to the end of a string, attach the string to the center of the protractor (see fig. 2). This device will enable you to determine the angle of inclination. Now all you need is the baseline b, the distance between the launch pad and where you stand when you sight on the rocket.

When the rocket reaches its maximum altitude, view the rocket along the edge of the protractor. Have your lab partner observe the angle indicated by the string. Because the protractor is inverted, this angle must be subtracted from 900 to obtain the angle of inclination. To find the altitude of the rocket, simply multiply the tangent of the angle of incidence by the length of the baseline.

### 3. Measuring the Moon

Key Concept
Students will use the concept of similar triangles to indirectly measure the diameter of the moon.

Teacher’s Instructions
This activity can be done in the classroom, if the moon happens to be visible from your windows, or it can be done at home by each student.

Student Instructions:
You may find this hard to believe, but you can measure the diameter of the moon from the comfort of your home.  The equipment needed includes an index card, a pin, two strips of opaque tape (masking or electrical tape works well), and a centimeter ruler.

Oh, and one other thing, you’ll need to know that the moon is 3 x 105 km from earth.

When the moon is full, place the two strips of tape 2cm apart on a windowpane facing the moon. After making a pinhole in the index card, observe the moon through the pinhole and two strips of tape. Back away from the window until the moon appears to just fill the space between the two strips of tape. Measure the distance from the card to the window. Using the proportionality of sides that exists for similar triangles (see figure above), calculate the diameter of the moon.

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### 4. Measuring Molecular Monolayers

Key Concept
Students will use the volume of a large number of items and the area covered by a single layer of those items to indirectly find the diameter of a single item.

Teacher’s Instructions
As with any lab using chemicals and glassware, this lab requires appropriate safety measures, such as goggles.  The final result for the height of an oleic acid molecule might not be very accurate, but the exercise is still worthwhile.  When students are able to measure something that they cannot see, they understand a bit more about how scientists work.

Student Instructions
Suppose you wanted to find the diameter of a BB, but didn’t have an instrument, such as a micrometer, suited for the job. What could you do?

One way to obtain the diameter of a single BB requires the use of many BB’s. Begin by placing a large number of BB’s in a graduated cylinder. Record the total volume of BB’s. (In carrying out this measurement, you are making an assumption. Do you know what it is?)

Now spread the BB’s out in a circular pattern on a table. This results in a monolayer, a cylindrical volume whose depth is a single BB. Measure the diameter of this circle. Because you may have difficulty making a perfect circle, make this measurement a number of times and find the average diameter. Divide the diameter by two, and use this radius to find the area of the circle.

The diameter of one BB is the same as the height of the very flat cylinder you just made. We can use the area of the circle and the total volume of BB’s (measured earlier) to find the height. The volume of a cylinder is the area of its base times the height.

V = A x h
or
h = V/A

Note: 1 mL = 1 cm3.

If you have a micrometer, use it to check your answer.

Believe it or not, you can estimate the size of a single molecule using a similar approach. This time however, you will be dealing with a monolayer of molecules rather than a monolayer of BBs. To perform the experiment you’ll need a pizza pan, some chalk dust, an eyedropper, a 10-ml graduated cylinder, and oleic acid solution. The oleic acid solution is prepared by adding 5-ml oleic acid to 995-ml ethanol.

After filling the pizza pan with water, spread chalk dust over the surface of the water. Easy does it, for too much powder will hinder the spread of the oleic acid. Using an eyedropper, carefully add just one drop of the oleic acid solution to the center of the pan. The alcohol will dissolve in the water, but the oleic acid will spread out to form a nearly circular shape. As you did with the BBs, measure the diameter of this rough circle a number of times and find the average. Then find the area of the circle.

Remembering that you put a single drop of oleic acid solution on the surface of the water, you will have to determine the volume of acid in a single drop of solution. To do this, count the number of drops needed to occupy 1-ml in the graduated cylinder. Do this several times and take an average. The volume of a single drop is found by dividing 1-ml (=1 cm3) by the average number of drops in a cm3. The actual volume of oleic acid is only 0.005 of the volume of a drop (Why?). Multiple the volume of a single drop by 0.005 to obtain the volume of oleic acid.

Just as with BBs, you can now find the size of a single molecule by dividing this volume by the area of the circle.

What assumption are you making regarding the shape of an oleic acid molecule?

## Molecular Size and Mass Kit

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### 5. Measuring Short Time Intervals with a Stroboscope

Key Concept

Stroboscopes are instruments that allow the viewing of repetitive motion in such a way as to make the moving object appear stationary. Stroboscopes may also be used to measure short time intervals.

Teacher Instructions
Stroboscopes may either be mechanical or electronic. Mechanical, or hand-held, stroboscopes consist of a disk with equally spaced slits around its circumference. The disk is spun around a handle while the viewer looks at a moving object through the slits. Electronic stroboscopes consist of a light source whose flash rate is controlled electronically. The activities that follow can be used as individual student labs or as a large class demonstration.

Student Instructions

A stroboscope is able to “freeze” repetitive motions because it only permits viewing at specific times. For example, if we are only allowed to see an object each time it makes one complete rotation, the object will always appear to be in the same place, and hence stationary. If the viewing frequency is slightly greater than the object’s rotational frequency, the object will appear to drift backward because it will be seen before it is able to complete a complete rotation. Conversely, the object will appear to drift forward if its frequency of rotation is slightly greater than the viewing frequency. Most of us are familiar with the apparent forward and backward motion of wheel covers on cars when the imperceptible flashing of streetlights illuminates them.

To freeze motion with a mechanical stroboscope, the rate of rotation of the strobe disk is adjusted until the number of slits passing the eye of the viewer each second equals the rate of the repetitive motion. For example, a fan will appear stopped if the rate of viewing equals the rate of rotation of the fan.

When an electronic stroboscope illuminates a moving object in a darkened room, the object will only be seen when the strobe light is on. When the rate of flashing matches the rate of the repetitive motion, the object will appear stopped.

If the viewing rate obtained with either type of strobe is known, it’s possible to measure the short time required for one rotation of a fan (or for one vibration of a tuning fork, or any other repetitive motion). Here’s how to measure the frequency of a fan’s rotation with a hand-held stroboscope.

1. Put a distinguishing mark on one fan blade.
2. View the fan in motion through a rotating strobe disk.
3. Adjust the rate of rotation of the disc until the marked blade appears stationary.
4. To insure that the rate of viewing is synchronized with the motion of the object, a condition known as resonance, increase the rate of rotation of the strobe disk until you see two images of the blade. Reducing the rate of rotation of strobe disk until a single image is seen will guarantee resonance. (Why?)
5. Have your partner use a stopwatch to determine the time it takes for ten rotations of the strobe disk.
6. Divide the number of rotations of the strobe disk (10) by the time obtained in step 5. This equals the number of rotations of the strobe disk per second.
7. Multiply the number of open slits in the disk by the number of rotations per second. This will yield the number of slits per second.
8. Because your viewing rate was synchronized with the rate of the repetitive motion, the number of slits per second equals the frequency of the fan’s rotation.
9. The period, or time required for one complete rotation of the fan blade, is found by finding the reciprocal of the frequency. For example, if the frequency equals 20 rotations per second, the time required for one rotation is 1/20 second per rotation.

To measure the period of a repetitive motion, in this case, a tuning fork, using an electronic stroboscope:

1. Strike the tuning fork and view it with the strobe flashing.
2. Obtain resonance by adjusting the strobe’s flash rate and read the flash rate from the strobe’s tachometer.
3. Find the reciprocal of the flash rate to find the period of the motion.

## Pulsar Strobe Light

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## Digital Strobe Light

In Stock SKU: P2-9010
\$349.00