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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Blind Spot - To see, or not to see!

Blind Spot

 

To see, or not to see

The eye's retina receives and reacts to incoming light and sends signals to the brain, allowing you to see. There is, however, a part of the retina that doesn't give you visual information. This is your eye's blind spot. 

 

• One 3 X 5 inch (8 x 13 cm) card or other stiff paper
• A meterstick

 

(5 minutes or less ) Mark a dot and a cross on a card as shown.

(5 minutes or more)

Hold the card at eye level about an arm's length away. Make sure that the cross is on the right.

Close your right eye and look directly at the cross with your left eye. Notice that you can also see the dot. Focus on the cross but be aware of the dot as you slowly bring the card toward your face. The dot will disappear, and then reappear, as you bring the card toward your face.

Now close your left eye and look directly at the dot with your right eye. This time the cross will disappear and reappear as you bring the card slowly toward your face.

Try the activity again, this time rotating the card so that the dot and cross are not directly across from each other. Are the results the same?
 

The optic nerve carries messages from your eye to your brain. This bundle of nerve fibers passes through one spot on the light sensitive lining, or retina, of your eye. In this spot, your eye's retina has no light receptors. When you hold the card so that the light from the dot falls on this spot, you cannot see the dot.

As a variation on this blind spot activity, draw a straight line across the card, from one edge to the other, through the center of the cross and the dot. Notice that when the dot disappears, the line appears to be continuous, without a gap where the dot used to be. Your brain automatically "fills in" the blind spot with a simple extrapolation of the image surrounding the blind spot. This is why you do not notice the blind spot in your day-to-day observations of the world.

Using a simple model for the eye, you can find the approximate size of the blind spot on the retina.

Mark a cross on the left edge of a 3 x 5 inch (8 x 13 cm) card. Hold the card 9.75 inches (25 cm) from your eye. (You will need to measure this distance; your distance from the card is important in determining the size of your blind spot.)

Close your left eye and look directly at the cross with your right eye. Move a pen on the card until the
point of the pen disappears in your blind spot. Mark the places where the penpoint disappears. Use the pen to trace the shape and size of your blind spot on the card. Measure the diameter of the blind spot on the card.

In our simple model, we are assuming that the eye behaves like a pinhole camera, with the pupil as the pinhole. In such a model, the pupil is 0.78 inches (2 cm) from the retina. Light travels in a straight line through the pupil to the retina. Similar triangles can then be used to calculate the size of the blind spot on your retina. The simple equation for this calculation is s/2 = d/D, where s is the diameter of the blind spot on your retina, d is the size of the blind spot on the card, and D is the distance from your eye to the card (in this case, 9.75 inches [25 cm]).

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Bird in the Cage - Stare at color and see it change!

     

Bird in the Cage

 

Stare at color and see it change.

You see color when receptor cells (called cones) on your eye's retina are stimulated by light. There are three types of cones, each sensitive to a particular color range. If one or more of the three types of cones becomes fatigued to the point where it responds less strongly than it normally would, the color you perceive from a given object will change. 

 

• 4 white posterboards or pieces of paper
• Bright red, green, and blue construction or contact paper
• Small piece of black construction or contact paper, or black marking pen
• Scissors
• Glue or glue stick (if you are using construction paper)
• Adult help

 

(30 minutes or less)

Cut the same simple shape, such a bird or a fish, from each of the three colored papers. Glue each shape on its own white board. Leave one white board blank. Cut a small black eye for each bird or fish or draw one in with the marking pen. If you choose a bird as the shape, draw the outline of a birdcage on the blank board; if you choose a fish, draw a fishbowl, etc. (Be creative!) 
 

(15 minutes or more)

Place the boards in a well-lit area. (Bright lighting is a significant factor in making this Snack work well.)
Stare at the eye of the red bird for 15 to 20 seconds and then quickly stare at the birdcage. You should see a bluish-green (cyan) bird in the cage. Now repeat the process, staring at the green bird. You should see a reddish-blue (magenta) bird in the cage. Finally, stare at the blue bird. You should see a yellow bird in the cage. (If you used a fish, try the same procedure with the fish and the bowl.) 
 

The ghostly fishes and birds that you see here are called afterimages. An afterimage is an image that stays with you even after you have stopped looking at the object.

The back of your eye is lined with light-sensitive cells called rods and cones. Cones are sensitive to colored light, and each of the three types of cones is sensitive to a particular range of color.

When you stare at the red bird, the image falls on one region of your retina. The red-sensitive cells in that region start to grow tired and stop responding strongly to red light. The white board reflects red, blue, and green light to your eyes (since white light is made up of all these colors). When you suddenly shift your gaze to the blank white board, the fatigued red-sensitive cells don't respond to the reflected red light, but the blue-sensitive and green-sensitive cones respond strongly to the reflected blue and green light. As a result, where the red-sensitive cells don't respond you see a bluish-green bird. This bluish-green color is called cyan.

When you stare at the green bird, your green-sensitive cones become fatigued. Then, when you look at the white board, your eyes respond only to the reflected red and blue light, and you see a red-blue, or magenta, bird. Similarly, when you stare at a blue object, the blue-sensitive cones become fatigued, and the reflected red and green light combine to form yellow.

You can design other objects with different colored paper and predict the results. Try a blue banana! For smaller versions, you can use brightly colored stickers (from stationery, card, or gift stores) on index cards.

One classic variation of this experiment uses an afterimage to make the American flag. Draw a flag, but substitute alternating green and black stripes for the familiar red and white stripes, and black stars on a yellow field for the white stars on a blue field. For simplicity, you can idealize the flag with a few thick stripes and a few large stars. When you stare at the flag and then stare at a blank white background, the flag's afterimage will appear in the correct colors.

You may also want to experiment with changing the distance between your eyes and the completely white board while you are observing the afterimage. Notice that the perceived size of the image changes, even though the size of the fatigued region on your retina remains the same. The perceived size of an image depends on both the size of the image on your retina and the perceived distance to the object.

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