Your laptop computers is a source of polarized light.
To Do and Notice
Turn on the computer and make the screen as white as possible.
Look at the screen through the polarizer.
Rotate the polarizer. Notice that at some angles of rotation the screen goes black.
Notice that the polarization of the screen is on a diagonal, why do you think this is?
Hold the cassette box between the white screen and the polarizer. Rotate the polarizer until the screen goes black. Notice the colors in the plastic. Keep rotating the polarizer notice how the colors change.
Rotate the cassette box. Explore colors and how they change.
Hold the plastic fork between the screen and the polarizer. Squeeze the tines of the fork. Notice how the colors change.
What's Going On?
Laptop computer screens have to be thin and run on low power so they do not use cathode ray tubes like your thick, power hungry, home television. They use liquid crystal displays.
When linearly polarized light goes through certain liquid crystal materials the direction of polarization can be rotated. Even more important, an applied voltage can change the rotation of the liquid crystals and also change the rotation of the direction of polarization of the light. What this means is that two linear polarizers can be placed so that no light passes through them. Then a liquid crystal can be placed between these polarizers so that a voltage can rotate the molecules of the liquid crystal and so rotate the direction of polarization of the light and allow it to pass through the second polarizer producing a spot of light or pixel on the screen.
The light produced by a laptop is polarized.
The light is polarized on a diagonal so that people can wear polarizing sunglasses, which are polarized vertically, and still see the screen.
The clear plastic of the cassette box is made of long chain polymers which also change the polarization of light. The plastic changes the polarization of different colors of light by different amounts. Thus when polarized white light goes into the plastic of a cassette box different colors of the light rotate their polarizations by different amounts. The polarizer next to your eye sorts out these colors.
When you squeeze the tines on the fork you stress the plastic. The resulting strain or deformation in the plastic changes the alignment of the polymers and so changes the colors of the plastic when viewed through the polarizer.
Changing the thickness of the plastic or the orientation of the stretched polymers by rotating the cassette box will also change the colors.
Stressing the plastic of a fork will change the orientation of the polymers in the fork and change the direction of polarization of light passing through the plastic.
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Scientific Explorations with Paul Doherty
24 May 2000