PHO 101 Photonics Concepts
Three Rivers Community College ONLINE

Home Lab #12

Exploring Polarization

Equipment/Supplies

Theoretical overview

"Natural" or randomly polarized light contains electromagnetic waves with electric field vectors that vibrate in all directions, with the vibration direction varying randomly in time. Linearly (or plane) polarized light has an electric field that varies in one direction only. Linearly polarized light may be produced by a polarizing filter (Polaroid® material), by scattering from molecular sized scatterers, by reflection from a nonconductive surface, and by double refraction (passing light through a birefringent material).

Procedure

Your lab report will be the answers to the QUESTIONS that appear in the procedure.:

1. Find the transmission axis of the polarizers (Polarization by reflection): To determine the transmission axes of the polarizers, you need to view the glare or shine from a nonconducting surface- in this case, the reflection from the surface of a bowl of water.

Place a bowl of water on a table or counter so that it reflects either the room lights or light from a window. Look at the reflection through one of the polarizing filters. (Remember to remove the plastic film from both sides, but don't throw it away.) Don't look directly down onto the surface, but view it from about a 50 degree incident angle.

While looking through the polarizer, rotate it and notice that the reflected light becomes dimmer and brighter. (Imagine that you are turning the "picket fence" so that the openings are horizonatal or vertical.) When you can see the reflection at its brightest, the transmission axis of the polarizer is horizontal. (The reflection from the water is polarized parallel to the water surface, or horizontally.) Mark the polarizer with a piece of tape so you know which direction is the transmission axis.

Repeat with the other polarizer.

2. Polarizer pair: Now that you have labelled the transmission axis of each polarizer, look through one of them at a source of light (a lamp is fine). The light you see passing through the polarizer is linearly polarized! You can change the direction of polarization by rotating the plastic. Notice that a lot of the light is blocked- by itself, an ideal polarizer removes 50% of the light. Your polarizer removes even more.

If you rotate this polarizer, the direction of polarization of the light reaching your eye change. Of course, you can't see this because the human eye cannot detect the direction of polarization.

Now hold the polarizer so that it is producing vertically polarized light (the transmission axis is vertical). Place the second polarizer in front of the first with its transmission axis also vertical. Without moving the first polarizer, rotate the second through 360 degrees as you look at the lamp through both polarizers. (You are aligning and misaligning the "picket fences" of the two polarizers.)

1. QUESTION: How many times does the light dim during the 360 degree rotation?

2. QUESTION: How are the transmission axes of the two polarizers oriented when the light dims? (what is the angle between them?)

3. Polarization by scattering: When light is scattered from very small scattering centers (molecular size), the scattered light is polarized. (This is called Rayleigh scattering.)The easiest molecular scatterer to find is- air! On a sunny day, observe the blue sky while looking through one of the polarizers. Look at a patch of sky AWAY from the sun, and rotate the polaroid in front of your eye as you look through it. Try looking at other parts of the sky as well. DON'T LOOK AT THE SUN.

3. QUESTION: What do you see as you rotate the polaroid?

Bees and other insects use the polarization of the sky to navigate (their eyes are adapted to sense polarization). They have to "stay home" if the sky is cloudy!

4. Stress patterns in plastic (changing the direction of polarization) Look through the two polaroids toward a lamp and turn them so the transmission axes are at 90 degrees (no light gets through). Place one of the pieces of plastic film that covered the polarizers between the "crossed" polarizers.

4. QUESTION: What do you see? What does the plastic film do to the polarized light passing through the first polarizer?

While looking through the two crossed polarizers, stretch the plastic film.

5. QUESTION: What do you see when the plastic is stretched? (The amount the plastic alters the polarization direction is wavelength dependent!)

Place other plastic transparent objects between the polarizers- a clear ruler (twist it and see what happens), or comb or protractor. Try your eyeglasses, if you wear them. Any stresses in the plastic can be visualized in polarized light. If a model of a structure (such as a bridge) is built of transparent plastic it can be weighted to simulate actual operating conditions. Viewing the model in polarized light helps to visualize where the structure will tressed.

5. (Optional) Rayleigh scattering in a fish tank: Rayleigh scattering does more than produce polarized light. It also explains why the sky is blue! Molecular (Rayleigh) scattering is wavelength dependent- when a beam of white light passes through air, the short wavelengths are scattered in all directions, and the long wavelengths continue more or less in a straight line. You can observe this in a fish tank. If you don't have fish in the tank, add a few drops of milk to enhance the scattering. (I don't know how fish would react to milk!)

Hold a flash light so it shines through the water, down the long axis of the fish tank . If you look at the light beam in the water from the side of the tank, you might notice a bluish tinge. (Blue light is scattered to the sides.) What is even more noticeable is looking back into the flashlight from the end of the tank. You will see that the bulb appears to have an orange tint when viewed through the water, compared to looking at the same bulb in the air. The short wavelengths have been scattered out of the beam so what remains appears more red. Sunlight appears more "red" to us on the surface of the earth than it appears at the edge of the atmosphere. Blue light is scattered out of sunlight, making the sky blue and the sun yellow-ish.

Back to the side of the tank, look at the beam (from the side) through the polarizing filter. Rotate the filter and you will see the light dim and brighten. You won't see this effect if you look into the end of the tank, toward the flashlight.