GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY
Photoelastic stress in plastic protractor. Photoelastic stress in a plastic protractor is observed by placing it between co-aligned (middle panel) and crossed (bottom panel) linear polarizers. Built-in stress causes optical birefringence which is revealed as colored bands when illuminated by white light. Also shown (top panel) is the same protractor in un-polarized light.
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