Photoelastic stress patterns of drawing equipment, viewed using polarised light. The equipment includes two French curves (left), a protractor (top centre), a ruler (lower right) and a set square (bottom right). The plastic the equipment is made from is displaying coloured stress patterns. Areas of similar colour are under similar stress. In photoelastic studies, the direction and magnitude of stresses placed on a design can be calculated using plastic, scaled- down models.
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