GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY
Photoelastic stress, Photo 2 of 3. Photoelastic stress in a plastic protractor is observed by placing it between co-aligned (Photo 2) and crossed (Photo 3) linear polarizers. Built-in stress causes optical birefringence which is revealed as colored bands when illuminated by white light. Also shown (Photo 1) is the same protractor in un-polarized light. Photo 2 of 3.
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