image credit: Wikimedia Commons; image source; larger image

Polarization and Interference Colors

-- For this photo, the protractor was backlit, and the light passed through two crossed polarizers, one behind the protractor and one in front.
-- Stress within the protractor affects the phase of the polarized light, producing cancellation of some wavelengths and reinforcement of others, and the result is the pattern of colors that you see.
-- Materials like the clear plastic of the protractor are called birefringent. To learn more about them, see the Hyperphysics pages Polarization and Interference Colors and Birefringent Materials.

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CoolStuff Newsletter: Polarization

Visit CoolStuff Newsletter: Polarization to learn more about polarization and birefringence, and for related activities.

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image credit: Jie Zhang/Duke University; image source; magnified view

Shear Forms Force Chains

-- You are looking at a thin box filled with small plastic discs. These discs are squeezed top-to-bottom and allowed to expand right-to-left. They are backlit, and the light passes through two linear polarizers, one behind the discs and one in front. The colors indicate the stresses inside the discs.
-- To learn more about this experiment, see the American Physical Society's Physical Review Focus article Shear Forms Force Chains. See this video to watch the experiment in action.

Birefringence

For more on birefringence and polarization, see the Davidson page Birefringence and the Hyperphysics page Birefringence.