When a polariser and analyser are mounted in the ‘crossed’ position, no light is normally transmitted through the combination. But, if a birefringent crystal is inserted between the ‘crossed’ polarisers, some light is transmitted.
Some materials such as glass and various plastics become birefringent when subjected to mechanical stress. This is the basis of photoelasticity. Stresses in pieces of engineering can be investigated by making plastic models and subjecting them to stress when they are inserted between ‘crossed’ polarizers. Very complicated stress distributions that are practically impossible to model mathematically may be studied optically using this technique.
The existance of the Kerr effect makes a ‘light valve’ possible. This is exploited in liquid crystal displays (LCDs).