Bubble Suspension - Soap bubbles float on a cushion of carbon dioxide gas

   

Bubble Suspension

 

Soap bubbles float on a cushion of carbon dioxide gas

This beautiful experiment illustrates the principles of buoyancy, semipermeability, and interference 

 

• A small aquarium
• Dry ice
• Bubble solution You can use a commercial solution like Wonder Bubbles™, or use the Exploratorium's recipe: 2/3 Cup (160 ml) Dawn™ dishwashing liquid and 1 tablespoon (15 ml) glycerine (available at most drugstores) in 1 gallon (3.8 l) of water. Aging the solution for at least a day before use significantly increases the lifetime of the bubbles.
• Gloves
• Adult help

 

(5 minutes or less)

Place a slab of dry ice flat in the bottom of the aquarium. (CAUTION: Use gloves when handling the dry ice; do not touch it with bare skin.) Allow a few minutes for a layer of carbon dioxide gas to accumulate. 
 

(15 minutes or more)

Blow bubbles so they float down into the aquarium. The bubbles will descend and then hover on the denser layer of carbon dioxide gas. After a few minutes, notice that the bubbles begin to expand and sink. Notice the color bands on the bubbles. Notice how some of the bubbles freeze on the dry ice. 
 

As dry ice turns from a solid to a vapor, or sublimes, it produces carbon dioxide gas. Carbon dioxide is denser than air. (Carbon dioxide molecules have an atomic mass of 44 amu [atomic mass units]. Air is made up of nitrogen, 28 amu, and oxygen, 32 amu.) The denser carbon dioxide gas forms a layer on the bottom of the aquarium.

A bubble is full of air. It floats on the carbon dioxide layer just like a helium balloon floating in the air. You might expect that the air in the bubble would cool and contract near the dry ice, but the bubble actually expands slightly. The soapy wall of the bubble allows carbon dioxide to pass through but does not allow air molecules to pass through. Initially, the concentration of carbon dioxide gas is low inside the bubble and high outside the bubble.

The gas gradually diffuses into the bubble, a process called osmosis. The bubble film is a semipermeable membrane--a surface that allows some substances to pass through while preventing others from passing through at all. The cells in your body have the same property. Water, oxygen, and carbon dioxide easily enter some cells, whereas other molecules do not. The added carbon dioxide makes the bubble denser, causing it to gradually sink. The carbon dioxide at the bottom of the tank is cold enough to freeze the bubble.

You can do many experiments with these bubbles.

What happens when bubbles of different sizes collide? Sometimes they make a single larger bubble, other times they join as two bubbles with a flat or bulging wall between them. If the two bubbles are the same size, the wall is flat between them, since the pressure is equal on both sides. If the two bubbles are of different sizes, the wall will bulge away from the smaller of two bubbles, since the smaller bubble will have a higher pressure inside.

How does a bubble respond to a comb that has been charged by rubbing it with a wool cloth? The neutral bubble is electrically polarized by, and attracted to, the charged comb.

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Bronx Cheer Bulb

Bronx Cheer Bulb

 

Some light sources may appear to wiggle and flash when you give them the raspberry, but the only thing wiggling is you

Some light sources flash on and off many times a second. When you give them the "Bronx cheer," you can see the their hidden flickering. 

 

• A digital radio or clock radio that uses light-emitting diodes (LEDs) These have red numbers. Or
• A circuit tester with an LED on it Or
• A neon glow lamp available from your local hardware store, such as a GE Guide lamp, and an extension cord. (Any nightlight labeled "1/4 watt" has a neon glow lamp in it)

 

(5 minutes or less)

No assembly is required for the digital radio, circuit tester, or neon glow lamp; just plug them in and observe them from a few feet away.

A simple source for a neon glow lamp is a button-type nightlight. These are small orange night-lights advertised as 1/4 watt bulbs. They do not have a regular replaceable small lighibulb. Plug the nightlight into the wall or into an extension cord that is plugged in. 
 

(5 minutes or more)

Observe the light source from 3 to 10 feet (90 to 300 cm) away and give it the "Bronx cheer." (A Bronx cheer, also known as a "raspberry," is a rude noise made by blowing air through your lips in a way that makes them vibrate.) Notice that the light seems to wiggle back and forth and flicker. Try shaking your head rapidly and notice whether the light still flickers. See if you can find other body motions that make the light flicker. Try the Bronx cheer on other light sources, such as incandescent lightbulbs. Notice whether the light flickers. 
 

No part of the LEDs or the neon glow tube move when you give the Bronx cheer. Instead, your whole body is vibrating, including your eyes. you can feel this vibration by putting your hand on your head as you blow. The LEDs flash on and off sixty times a second (a neon glow tube glows on and off 120 times a second). This flashing is so fast that your eyes normally can't separate the "blinks." But when your body is vibrating, your eyes are in a different position each time the bulb flashes. As the image of the bulb traces a path across your eyes, it looks like the bulb is moving and flickering. An incandescent bulb won't flicker when you give the Bronx cheer, because the bulb doesn't flash on and off. Incandescent bulbs give a steady glow.

Plug a commercial neon night-light into an extension cord. Tape it firmly in place. Twirl the light around in a circle. (Be careful not to let it hit anything.) Notice that you can see the light flashing. Since the light is moving, it's in a new position each time it flashes. The light traces a path across your eye, and its flashes become spread out and visible. Find an oscilloscope and set it up so that the beam goes straight across the middle of the screen in about 1/1ooth of a second. Ask a couple of friends to stand back a few yards from the scope. Tell them that the oscilloscope is an eating detector. Have your friends watch the scope at the same time. Have one of them eat a peanut and the other one not eat. The person eating the peanut will see the beam jump up and down. Eating causes vibrations of your skull, including vibrations of your eyes. If your eyes are moving, the dot of light scanning across the oscilloscope shines on different parts of your eyes and appears to jump around.

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Bridge Light - A thin layer of air trapped between two pieces of Plexiglas™ produces rainbow-colored interference patterns

   

Bridge Light

 

A thin layer of air trapped between two pieces of Plexiglas™ produces rainbow-colored interference patterns

When light hits two slightly separated transparent surfaces, part of the light will be reflected from each surface. If the distance between the surfaces is a multiple of half or whole wavelengths of the light, constructive and destructive interference will occur, producing an interference pattern. 

 

• 2 sheets of Plexiglas™, 1/4 or 1/8 inch (.64 or .33 cm) thick and approximately 1 foot (30 cm) square. (Size is not critical.)
• 1 piece of dark construction paper
• One 3 x 5 inch (8 x 13 cm) piece of transparent red plastic
• Electrical or duct tape
• A light source, such as a desk lamp

 

(15 minutes or less)

Peel the paper from the Plexiglas™ and smooth off all edges with sandpaper if necessary. Be careful not to scratch the surfaces. Clean the top and bottom surfaces with alcohol and a soft cloth. Press the plates tightly together and tape around the edges to hold them in place. Tape a sheet of dark construction paper to one plate to make the interference patterns more visible. 
 

(15 minutes or more)

Hold the plates, with the dark-paper side on the bottom, in any strong source of white light. Observe the rainbow-colored interference patterns. The patterns will change as you bend, twist, or press on the plates. Notice that the patterns strongly resemble the contour lines on a topographic map.
Place the red plastic between the light source and the plates. Notice that the patterns are now just red and black. 
 

Light waves reflect from the surfaces of two plastic sheets separated by a thin air gap. These light waves meet after reflecting from the two surfaces. When two waves meet, they can add together, cancel each other, or partially cancel each other. This adding and canceling of light waves, called constructive interference and destructive interference, creates the rainbow-colored patterns that you see.

White light is made up of all different colors mixed together. When light waves of a particular color meet and cancel each other, that color is subtracted from white light. For example, if the blue light waves cancel, you see what is left of white light after the blue has been removed--yellow (the complementary color of blue).

The thickness of the gap between the plates determines which colors of light cancel out at any one point. For example, if the separation of the plates is roughly equal to one-half the wavelength of blue light (or some multiple of it), the crests of waves of blue light reflected from the top surface of the air gap will match up with the troughs of waves reflected from the bottom surface, causing the blue light to cancel out.

This is what happens: Imagine that the distance between the two plates is one-half the wavelength of blue light. When a wave hits the top of the air layer, part reflects and part continues on. Compared to the part that reflects from the top of the air layer, the part that continues on and reflects from the bottom travels an extra wavelength through the air layer (half a wavelength down and half a wavelength back). In addition, the wave that reflects from the bottom is inverted. The net effect is that the blue light waves reflected from the two surfaces recombine trough-to-peak, and cancel each other out.

Because the interference pattern depends on the amount of separation between the plates, what you're actually seeing is a topographical map of the distance between plates.
When you place a red filter in front of the light source, only red and black fringes will appear. Where destructive interference takes place, there is no red light left to reach your eyes, so you see black. Where the waves constructively interfere, you see red.

If you can find a sodium-vapor lamp (a yellow street lamp, for example), try placing the plates under its light. The sodium vapor gives off sodium's predominant fingerprint: a very pure yellow light.

The beautiful rainbow colors you see in soap bubbles and on pieces of metal heated to high temperatures are produced in the same way: by light reflecting from the top and bottom of a thin transparent layer.

When you open a package of new, clean microscope slides, you can often see colored interference patterns created by the thin air space between the glass slides

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Bone Stress - Polarized light reveals stress patterns in clear plastic!

    

Bone Stress

 

Polarized light reveals stress patterns in clear plastic

When certain plastics are placed between two pieces of polarizing material, their stress patterns become dramatically visible in a brightly colored display. A stressed plastic object can be used to illustrate stresses found in bones. 

 

• Overhead projector and screen
• 2 polarizing filters (If polarizing material is not readily available, you can use two lenses from an old pair of polarizing sunglasses)
• A transparent plastic picnic fork, or thin pieces (about l/16 to l/s inch [.16 to .33 cm]) of transparent plastic (Plastic from cassette tape cases works well)

(15 minutes or less)

Set up your overhead projector so that the light shines on the screen.

Place one of the filters on the stage of the overhead projector. If the second piece of polarizer is large enough to cover most of the lens on the arm of the projector, then tape it there. (See drawing.)

If you are using the lens from a pair of sunglasses, then devise a stand to hold the lens a few inches above the stage of the projector, right over the first filter.

If you are using thin plastic, such as the plastic from a cassette tape case, cut it into the shape of letters that can be flexed (C, J, S, K, etc., or any other shape that can be flexed). 
 

(5 minutes or more)

Hold the fork or plastic letter above the first filter and below the second filter. Induce stress by squeezing the tines of the fork together or deforming the letter. Notice the colored stress pattern in the image of the plastic that is projected on the screen. Try rotating one of the polarizing filters. Some orientations will give more dramatic color effects than others. 
 

The first polarizing filter limits the vibration of light waves to one plane --- that is, it polarizes the light.

The white light of the overhead projector is made up of light of all colors. The plastic breaks the light waves that make up each color into two perpendicularly polarized waves. These two waves travel through the plastic at different speeds, which are determined by the light's color. When the two waves meet and recombine, they produce a polarization unique to that color. The direction of polarization determines whether light of a certain color can pass through the second polarizing filter. If the new direction of polarization lines up with the second filter, light of that color passes through the filter and you see it. If the new direction of polarization does not line up with the second filter, light of that color is blocked. By rotating the filter, you can let different colors pass through, and the colors you observe will change.

Stressing the plastic alters its structure, which affects how rapidly light of different polarizations travels through the plastic. Where colored patterns change rapidly, stress is high. Where colored regions are spread out and change gradually, stress is low. Sharp corners, or areas that have been cut or stamped, are usually areas of stress concentration. Changing the stresses in the plastic will change the color pattern in the plastic.

Stress patterns and concentrations like the ones visible in the plastic are also present in your bones, as they flex under the daily loads imposed upon them.

The college editions of Conceptual Physics by Paul Hewitt (HarperCollins College Publishers, New York, 1993) contain an excellent diagram and explanation of the formation of colors by polarized light traveling through plastic or similar material. You will need to have a basic understanding of vectors to read this material. For related information, see the PolarizedLight Mosaic Snack.

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Blue Sky - Now you can explain why the sky is blue and the sunset is red!

    

Blue Sky

 

Now you can explain why the sky is blue and the sunset is red

When sunlight travels through the atmosphere, blue light scatters more than the other colors, leaving a dominant yellow-orange hue to the transmitted light. The scattered light makes the sky blue; the transmitted light makes the sunset reddish orange. 

 

• A transparent plastic box, or a large beaker, jar, or aquarium
• A flashlight or projector (either a slide or filmstrip projector)
• Powdered milk
• Polarizing filter (such as the lens from an old pair of polarized sunglasses)
• Blank white card for image screen
• Paper hole-punch
• Optional: Unexposed (black) 35 mm slide or photographic film, or an index card cut to slide size

 

(15 minutes or less)

Fill the container with water. Place the light source so that the beam shines through the container. Add powdered milk a pinch at a time; stir until you can clearly see the beam shining through the liquid.

(15 minutes or more)

Look at the beam from the side of the tank and then from the end of the tank. You can also let the light project onto a white card, which you hold at the end of the tank. From the side, the beam looks bluish-white; from the end, it looks yellow-orange.

If you have added enough milk to the water, you will be able to see the color of the beam change from blue-white to yelloworange along the length of the beam.

If you want to look at a narrower beam of light, use a paper hole-punch to punch a hole in the unexposed black slide or in a piece of 35 mm film, or even in an index card cut to size. Place the slide, film, or index card in the projector. (Do not hold it in front of the lens.) Focus the projector to obtain a sharp beam. 
 

The sun produces white light, which is made up of light of all colors: red, orange, yellow, green, blue, indigo, violet. Light is a wave, and each of these colors corresponds to a different frequency, and therefore wavelength, of light. The colors in the rainbow spectrum are arranged according to their frequency: violet, indigo, and blue light have a higher frequency than red, orange, and yellow light.

When the white light from the sun shines through the earth's atmosphere, it collides with gas molecules. These molecules scatter the light.


The shorter the wavelength of light, the more it is scattered by the atmosphere. Because it has a shorter wavelength, blue light is scattered ten times more than red light.

Blue light also has a frequency that is closer to the resonant frequency of atoms than that of red light. That is, if the electrons bound to air molecules are pushed, they will oscillate with a natural frequency that is even higher than the frequency of blue light. Blue light pushes on the electrons with a frequency that is closer to their natural resonant frequency than that of red light. This causes the blue light to be reradiated out in all directions, in a process called scattering. The red light that is not scattered continues on in its original direction. When you look up in the sky, the scattered blue light is the light that you see.

Why does the setting sun look reddish orange? When the sun is on the horizon, its light takes a longer path through the atmosphere to your eyes than when the sun is directly overhead. By the time the light of the setting sun reaches your eyes, most of the blue light has been scattered out. The light you finally see is reddish orange, the color of white light minus blue.

Violet light has an even shorter wavelength than blue light: It scatters even more than blue light does. So why isn't the sky violet? Because there is just not enough of it. The sun puts out much more blue light than violet light, so most of the scattered light in the sky is blue.

Scattering can polarize light. Place a polarizing filter between the projector and the tank. Turn the filter while one person views the transmitted beam from the top and another views it from the side. Notice that when the top person sees a bright beam, the side person will see a dim beam, and vice versa.

You can also hold the polarizing filter between your eyes and the tank and rotate the filter to make the beam look bright or dim. The filter and the scattering polarize the light. When the two polarizations are aligned, the beam will be bright; when they are at right angles, the beam will be dim.

Scattering polarizes light because light is a transverse wave. The direction of the transverse oscillation of the electric field is called the direction of polarization of light.


The beam of light from the slide projector contains photons of light that are polarized in all directions. horizontally, vertically, and all angles in between. Consider only the vertically polarized light passing through the tank. This light can scatter to the side and remain vertically polarized, but it cannot scatter upward! To retain the characteristic of a transverse wave after scattering, only the vertically polarized light can be scattered sideways, and only the horizontally polarized light can be scattered upward. This is shown in the drawing.

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