PAUL SHAMBROOM / SCIENCE PHOTO LIBRARY PAUL SHAMBROOM / SCIENCE PHOTO LIBRARY
Photoelastic effects in a thin polymer film, showing an engineer using a pair of callipers to measure the width of a stress pattern in the film. Polarised light is used to visualise residual stress in some types of plastic. This phenomenon, called photoelasticity, is used in stress calculations in engineering design by building plastic scale models of a subject (a bridge, for example) and examining the stress pattern produced by polarised light.
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