How Things Work - Chapter 14 Demonstrations
Section 14.1 Sunlight
Demonstration 14.1.1: Sunspots Aren't Really Dark
Description: The filament of an uncoated light bulb appears dark against the bright background of a light box or an overhead projector's light source. But when the light box or overhead projector are turned off, the filament is glowing reasonably brightly—just not as brightly as the background was.
Purpose: To show that while sunspots are relatively cool spots on the sun's surface, they are still very hot and are still radiating lots of thermal energy.
Supplies:
1 clear light bulb
1 variable-voltage transformer (an autotransformer such as a Variac)
1 light box or overhead projector
Procedure: Place the clear light bulb in front of the light box or on the overhead projector. Its filament will appear dark against the bright background. Now connect the bulb to the variable-voltage transformer and begin to turn up the current through the bulb. Eventually, the filament will appear about as bright as the background. Reduce the current so that the bulb appears noticeably darker than the background. Finally, turn off the light box or projector and observe that the filament is actually still glowing brightly. It just wasn't as bright as the background.
Explanation: Sunspots are relatively cool regions on the sun's surface. They appear dark only because they aren't as bright as the surrounding sun surface, but they are still extremely bright sources of light.
Demonstration 14.1.2: The Blue Sky and the Red Sunset
Description: You shine light from a slide projector through a tank of clear water. A bright, white circle appears on the screen beyond. But after you add a chemical to the water, the circle gradually reddens, like the setting sun. The colors of light scattered by the water also change gradually from blue, like the sky, to various shades of purple, like those visible at sunset.
Purpose: To show why the sky is blue and why sunsets are red.
Supplies:
1 aquarium tank
1 slide projector
1 projection screen
1 cardboard "slide" with a circular hole
water at room temperature
sodium thiosulfate ("hypo")
sulfuric acid
1 stirring stick
Procedure: Dissolve 15 ml (1 tablespoon) of sodium thiosulfate (a white powder) into 16 liters of water in the aquarium. Stir to dissolve the powder. Insert the cardboard circle slide into the slide projector and direct the projector's beam of light through the long direction of the aquarium and onto the screen. You should see a bright, white disk and the water should look essentially clear. Now add 50 ml of sulfuric acid to the aquarium and stir to mix. Allow the water to settle for a few seconds and everything should look as it was.
But about 2 minutes later, the water will begin to have a blue appearance—tiny transparent particles will be forming in it that will Rayleigh scatter blue light. The disk on the screen will begin to look yellowish. By about 3½ minutes, the disk will look decidedly yellow and by about 4½ minutes, it will be full red. Throughout this period, the colors of light scattering from the aquarium will progress from blue to various shades of purple and pink, just as at sunset.
The recipe scales, so that if your aquarium needs more or less water, just scale the sodium thiosulfate and sulfuric acid accordingly. To make the reaction proceed faster, increase the concentrations of the two chemicals. If you double both of them, everything will happen roughly 4 times as fast. Be sure to dispose of the acidic solution properly when you're done.
Explanation: The chemical reaction that occurs in the water forms tiny clear particles that gradually grow in size. As they do, they become more and more effective at Rayleigh scattering light. While the particles are very small, blue light is scattered more effectively than red light, so the red light makes it through the aquarium to the screen and the blue light is scattered about the room.
Demonstration 14.1.3: Refraction
Description: A beam of light passes through glass or plastic surfaces and bends as is does.
Purpose: To demonstrate that refraction occurs when light changes speeds.
Supplies:
1 source of light rays
1 glass or plastic prism
Procedure: Show that a beam of light bends when it encounters a glass or plastic surface at anything but normal incidence. Show that light bends toward the normal as it enters glass or plastic, and bends away from the normal as it leaves.
Explanation: Light slows down as it enters glass, plastic, or water, and speeds up as it leaves. These changes in speed affect the propagation of light at the interfaces between materials and cause that light to bend.
Demonstration 14.1.4: Reflection
Description: A beam of light partially reflects from the surfaces of glass or plastic.
Purpose: To demonstrate that partial reflection occurs when light changes speeds.
Supplies:
1 source of light rays
1 glass or plastic sheet or rectangle
Procedure: Show that a beam of light partially reflects from each surface between air and glass or plastic.
Explanation: Light slows down as it enters glass, plastic, or water, and speeds up as it leaves. These changes in speed (or more generally, impedance mismatches) cause partial reflections of the light waves. For typical optical materials, the reflections are about 4% from each surface.
Demonstration 14.1.5: Dispersion
Description: A beam of light bends as it passes through a glass or plastic prism, but the different colors in the white light bend differently and a rainbow forms on the screen where the beam finally hits.
Purpose: To show that lights of different colors travel at different speeds in materials (dispersion) and thus experience unequal refraction.
Supplies:
1 source of white light rays
1 glass or plastic prism—60° angles, if possible
1 projection screen or another white surface
Procedure: Direct the beam of light through the prism at a shallow angle so that the light bends severely on both entry and exit from the prism. Have the beam then impinge on the projection screen. The violet portions of the spectrum will bend more than the red portions, so the light will appear as a rainbow on the screen.
Explanation: The charges in most materials respond more easily to high frequency, short wavelength light (violets) than they do to low frequency, low wavelength light (reds). As a result, violet light slows down more in materials than does red light. Violet light thus experiences more refraction than red light and the colors travel different paths to the screen.
Demonstration 14.1.6: Polarizing Glasses
Description: You show that glare, the sunlight reflected at shallow angles by horizontal surfaces, is mostly horizontally polarized. Polarizing sunglasses block this glare rather effectively.
Purpose: To show why polarizing sunglasses are helpful at blocking sunlight that reflects upward from horizontal surfaces.
Supplies:
1 light source (a slide projector)
1 glossy non-metallic horizontal surface, such a table with a layer of clear glossy varnish
1 large polarizing sheet
1 pair of polarizing sunglasses
Procedure: Direct the light source so that it reflects at a shallow angle from the glossy horizontal surface. Place the polarizing sheet in the reflected light and show that most of the light is blocked when the polarizing sheet is turned to absorb horizontally polarized light. Show that the sunglasses are also built to absorb horizontally polarized light and thus to block glare of this type.
Explanation: Light partially reflects from transparent materials, but the fraction of light that reflects depends on the light's polarization. If the light is polarized across the surface (e.g., horizontally polarized light hitting a horizontal surface), the reflection is stronger than if the light were polarized into the surface (e.g., vertically polarized light hitting a horizontal surface). Polarizing sunglasses recognize this preferential reflection of horizontally polarized light from horizontal surfaces. By blocking all horizontally polarized light, they eliminate most glare.
Demonstration 14.1.7: Soap Bubbles and Interference
Description: You blow soap bubbles on the surface of a sheet of white plastic laying on an overhead projector. The soap bubbles exhibit beautiful colors in a darkened room.
Purpose: To show interference effects in soap bubbles.
Supplies:
1 overhead projector
1 sheet of thin white plastic or a plastic diffuser
soap bubble mix
1 drinking straw or bubble wand
Procedure: Place the plastic sheet on the surface of the overhead projector and turn on the projector. Wet the surface of the plastic sheet with bubble solution and blow one or more large bubbles on its surface. In a darkened room, the bubble will appear brightly colored.
Explanation: Light waves partially reflect from each surface of the soap film and can bounce about inside the film several times before emerging. When these partial waves join together, they may interfere destructively or constructively, depending on their wavelengths and on how far they have traveled through the soap film before joining together. The partial waves of some wavelengths will join together in constructive interference and these wavelengths will appear bright. Other wavelengths will experience destructive interference and won't be visible. Since only some wavelengths experience constructive interference, the soap film appears brightly colored. The film tends to be thicker at the bottom than at the top, so the colors vary with position.
Demonstration 14.1.8: The Polarization of the Blue Sky
Description: While the Blue Sky and Red Sunset demonstration is proceeding (see above), you hold a polarizing sheet in front of the aquarium tank and observe that the blue light emerging from the water is mostly vertically polarized.
Purpose: To show that blue light from the sky is somewhat polarized.
Supplies:
1 aquarium tank
1 slide projector
1 projection screen
1 cardboard "slide" with a circular hole
water at room temperature
sodium thiosulfate ("hypo")
sulfuric acid
1 stirring stick
1 polarizing sheet
Procedure: Repeat the Blue Sky and Red Sunset demonstration (or perform this demonstration at the same time as that other demonstration—a little tricky, unfortunately). Hold the polarizing sheet on the side surface of the aquarium and show that the blue light you see through it is much brighter when the sheet is permitting vertically polarized light to pass than when it's permitting horizontally polarized light to pass.
Explanation: Photons of vertically polarized light from the slide projector are much more likely to scatter 90° in the horizontal plane than are horizontally polarized photons. That's because a vertically polarized photon causes vertical oscillations of charges in the particles in the water and these vertical oscillations of charge radiate vertically polarized waves into the horizontal plane. A horizontally polarized photon from the slide projector will make charges in the particles oscillate horizontally, and those charges won't radiate at all in the direction of the polarizing sheet.
Demonstration 14.1.9: White Sugar and Clear Rock Candy
Description: You show that while large sugar crystals (rock candy) appear clear, a pile of tiny sugar crystals appears white.
Purpose: To show that when light travels through many interfaces between clear materials, the light is reflected and scattered, and the materials appear white.
Supplies:
granulated sugar
rock candy
Procedure: Show that the rock candy is clear and that the granulated sugar is white, despite the fact that both are the same chemical. In fact, you might crush the rock candy to make white granulated sugar.
Explanation: Each time light travels from air into sugar or from sugar into air, some of that light reflects. While these transitions occur only occasionally in rock candy, they occur frequently in a pile of granulated sugar. With enough reflections from randomly oriented sugar surfaces, the pile of granulated sugar appears white.
Section 14.2 Discharge Lamps
You might revisit the fluorescence demonstration from Section 11.2.
Demonstration 14.2.1: The Spectrum of an Incandescent Filament
Description: The vertical filament of a long incandescent lamp emits yellowish-white light. A CCD camera views the filament through a diffraction grating and displays a rainbow sweep of colors, mostly in the reds, oranges, and yellows.
Purpose: To show the spectral lines of a thermal light source.
Supplies:
1 clear incandescent light bulb with a single, vertical filament (can be a halogen lamp).
1 CCD color camera and monitor
1 transmission diffraction grating
Procedure: Mount the incandescent lamp vertically and turn it on. It will begin emitting yellow-white light.
To see the specific wavelengths of light, use the CCD camera to observe the filament through the diffraction grating. With the camera aimed properly to one side, you should see a rainbow sweep of colors from violets to reds across the screen of the monitor. Note that most of the light is in the red end of the spectrum.
Explanation: The hot filament emits a black-body spectrum at roughly 2500 °C. Most of that light is in the infrared. The violet end of this thermal spectrum is noticably weak.
Demonstration 14.2.2: The Spectrum of a Gas Discharge
Description: A vertical glass tube emits a bright line of light as an electric discharge occurs inside it. The colors of this discharge depend on the type of gas inside that tube. A CCD camera views the tube through a diffraction grating and displays a series of bright spectral lines.
Purpose: To show the spectral lines of a gas discharge.
Supplies:
1 set of gas discharge lamps and a high-voltage power supply
1 CCD color camera and monitor
1 transmission diffraction grating
Procedure: Mount one of the discharge lamps vertically in the power supply and turn it on. Note that the gas is producing light because its atoms are being excited by a stream of high-energy electrons. When one of these electrons collides with a gas atom, that atom may be shifted to an electronically excited state and may subsequently emit a photon of light. The wavelength and color of that light are determined by the atomic structure of the atom.
To see the specific wavelengths of light, use the CCD camera to observe the discharge through the diffraction grating. With the camera aimed properly to one side, you should see a series of spectral lines that sweep from violets to reds across the screen of the monitor. (If you need to attenuate the light from the tube, try two crossed polarizers.) Turn off the high-voltage supply and change the tube to one with a different gas. Show that the spectral lines are unique to the particular gas.
Explanation: The electronic structures of the different atoms depend on the charges of their nuclei and, consequently, to the numbers of electrons they have. In the dark, electronically excited atoms tend to emit their excess energy as light and return to their ground electronic states.
Demonstration 14.2.3: The White Fluorescence of the Phosphors in a Fluorescent Lamp
Description: You exposed the phosphors from a fluorescent lamp to ultraviolet light and they glow with white light.
Purpose: To show how the phosphors in a fluorescent lamp convert the ultraviolet light from the mercury atoms into visible light.
Supplies:
1 ultraviolet lamp (ideally a short wavelength mercury lamp)
phosphors removed from a fluorescent lamp
Procedure: Collect the phosphors from inside a fluorescent lamp. (While there is a tiny amount of mercury trapped in these phosphors, they are otherwise non-toxic. If you like, you can eliminate the mercury by baking the phosphors gently in a well-ventilated area.) Expose these phosphors to ultraviolet light and observe that they emit white light.
Explanation: The phosphors are a mix of different materials that glow with a spectrum that mimics that of sunlight.
Follow-up: You could compare the lights from the four standard lamp styles: regular and deluxe cold and warm whites.
Demonstration 14.2.4: Different Fluorescent Fixtures
Description: You turn on several different types of fluorescent fixtures to show how they initiate and control their gas discharges.
Purpose: To show the various techniques for starting and sustaining the discharges inside fluorescent lamps.
Supplies:
1 manual preheat lamp (you must push one button—typically a red button—and the lamp starts when you release the button)
1 automatic preheat lamp (the lamp blinks on several times before it glows continuously)
1 rapid start lamp (it starts shortly after you turn it on, without blinking, and may be dimmed in some cases—if you can find a dimmable lamp, that's ideal)
1 instant start lamp (it starts immediately as you turn it on and its tubes have only one pin at each end)
Procedure: Demonstrate the 4 lamps one at a time (if you can find all of them).
The manual preheat lamp starts only after you release the preheat button. Note that as you press the preheat button, the ends of the tubes (the filaments) glow red hot. When the filaments are hot enough and you release the preheat button, the discharge will start when you release the button and the discharge will keep the filaments hot enough to provide the free electrons needed to sustain the discharge.
The automatic preheat lamp uses a starter device (often a little metal can) to do the same job that you would have done with the preheat button had it been a manual fixture. The filaments are first heated red hot and then the discharge is tried. The starter device usually tries to start the discharge several times before it actually starts properly. That's why the discharge blinks before becoming continuous.
The rapid start lamp continuously heats the filaments, both before and while the discharge is operating. When you turn on the lamp, the ballast immediately begins to heat the filaments and the discharge starts smoothly (though with a little flickering) as soon as the filaments are hot enough. Because the filaments are always kept hot enough to emit electrons, even when the discharge is weak, a rapid start lamp can be dimmed.
The instant start lamp uses high voltage to start the discharge operating. It turns on immediately.
Explanation: To sustain a discharge in a fluorescent tube, electrons must be emitted from its ends so that current can flow through the tube. The free electrons are normally released by hot electrodes (filaments) at the ends of the tube. There are different techniques for heating these electrodes.
Demonstration 14.2.5: A High-Pressure Sodium Vapor Lamp
Description: You turn on a high pressure sodium vapor lamp and watch the color of its light and its brightness evolve. It starts with a dim orange-violet glow and gradually develops a brilliant orange-white light.
Purpose: To show how the pressure of gas in a sodium vapor discharge affects the spectrum of light emitted by that discharge.
Supplies:
1 high pressure sodium vapor lamp (available from a hardware store)
1 CCD color camera and monitor
1 transmission diffraction grating
Procedure: Place the diffraction grating in front of the CCD camera and aim the CCD camera to one side of the high pressure sodium-vapor lamp so that it will observe the dispersed colors of the lamp. Now turn on the lamp. The discharge will begin as a dim violet-orange glow—the pressure of sodium atoms in the lamp is extremely low and you are seeing mostly light emitted by gases added to start the discharge. But as the lamp warms up and more sodium atoms enter the vapor, the lamp will begin to emit bright orange light. At still higher densities and pressures, the orange light will smooth out into an orange-white light. If you watch the spectrum evolve on the monitor of the CCD camera, you'll see that the bright orange line that first appears when the discharge has just begun to warm up becomes broader and broader and develops a dark center.
Explanation: At low pressures, the sodium atoms in a gas discharge emit mostly sodium resonance radiation at 590 nm. But at higher pressures and densities, the 590 nm light experiences radiation trapping—it goes from one atom to the next and is virtually unable to escape from the dense gas. However, light emitted by excited sodium atoms during collisions is shifted from its normal wavelengths and has a much better chance of escaping from the gas. That's why the 590 nm line broadens and develops a dark center—light right at 590 nm can't get out of the discharge, but collision-broadened light that isn't right at 590 nm can get out. Other spectral lines also appear as the density of sodium atoms increases and the lamp stops being such a monochromatic orange.
Demonstration 14.2.6: A High-Pressure Mercury Lamp
Description: You turn on a high pressure mercury lamp and watch the color of its light and its brightness evolve. It starts with a dim violet glow and gradually develops a brilliant blue-white light.
Purpose: To show how the pressure of gas in a mercury discharge changes the spectrum of light emitted by that discharge.
Supplies:
1 high pressure mercury lamp (available from a hardware store)
1 CCD color camera and monitor
1 transmission diffraction grating
Procedure: Place the diffraction grating in front of the CCD camera and aim the CCD camera to one side of the high pressure mercury lamp so that it will observe the dispersed colors of the lamp. Now turn on the lamp. The discharge will begin as a dim violet glow—the pressure of mercury gas in the lamp is low and the light it emits is mostly invisible ultraviolet; the same ultraviolet that's used in a fluorescent lamp. But as the lamp warms up and more mercury atoms enter the vapor, the lamp will begin to emit a rich spectrum of colors and its light will appear blue-white. Watch the monitor and see how new spectral lines continue to appear. The lamp probably contains metal-halides to improve its whiteness, so some of the lines are due to those added materials.
Explanation: At low pressures, the mercury atoms in a gas discharge emit mostly the mercury resonance radiation at 254 nm. But at higher pressures and densities, the 254 nm light experiences radiation trapping—it goes from one atom to the next and is virtually unable to escape from the dense gas. New spectral lines that are able to escape from the gas begin to appear, including many that correspond to forbidden transitions—transitions that can't occur in isolated atoms but that are allowed during collisions.
Section 14.3 Lasers and LEDs
Demonstration 14.3.1: An Open Helium Neon Laser
Description: You observe the spectral lines in the gas discharge of a helium neon laser (or a neon discharge lamp). Only one of these spectral lines is present in the laser light emerging from a helium-neon laser.
Purpose: To show that, of the spectra lines in the neon atom, only one of them is selected for amplification in a normal helium neon laser.
Supplies:
1 red helium-neon laser with an exposed discharge tube (if available; else a neon gas discharge lamp and a normal red helium-neon laser)
1 CCD color camera and monitor
1 transmission diffraction grating
Procedure: Turn on the helium-neon laser (or the gas discharge lamp) and observe its spectral lines with the CCD camera and the diffraction grating. Now look at the laser line and identify the spectral line in the neon discharge that's being amplified to form the laser line.
Explanation: The helium neon laser is amplifying light from neon's spectral line at 632.8 nm. An elevated population of excited neon atoms is being created by a collisional energy transfer from excited helium atoms and the excited neon atoms are amplifying this 632.8 nm light to form a laser beam.
Demonstration 14.3.2: The Coherence of Laser Light
Description: You observe the patterns produced when laser light passes through slits and screens.
Purpose: To show that because laser light is coherent, it exhibits dramatic interference effects.
Supplies:
1 continuous-wave visible laser
slits (single, double, etc.)
screens (fine mesh)
Procedure: Place the slits and screens in front of the laser beam, one at a time, and discuss the patterns that form on the wall beyond. Point out that such patterns normally don't form with spontaneous or thermal lights because these other forms of light don't have the coherence needed to exhibit strong interference effects.
Explanation: The photons leaving a laser are copies of one or a small number of original photons. Because of the identical character of the photons, they can interfere not only with themselves but also with one another. This broad flexibility with interference in laser beams makes it possible to see some remarkable effects.