The shattered plastic casing of a music tape cassette, showing the reel of magnetic tape contained within. When photographed through cross polarised light, coloured patterns are visible which correspond to stress areas in the plastic casing; areas of similar colour correspond to regions under a similar degree of stress. This phenomenon, known as photoelasticity, is used to predict stress forces in real objects, eg during the design of a motor vehicle axle. Transparent plastic models with loads proportional to those in the real object are made to determine the stress distribution.