Two effects in which the optical properties of transparent material are affected by electric or magnetic fields.

The electro-optical effect is the effect whereby the direction of polarization of plane-polarized light through a refractive medium is rotated by an electric field applied perpendicularly to the direction of propagation of the light. The Kerr cell utilizes this effect. It consists of two parallel plates immersed in a liquid that exhibits a marked Kerr effect. Polarized light passing through the cell can be interrupted by the application of an electric field. Pockel’s effect is the Kerr effect when it occurs in a piezoelectric material. Pockel’s effect can be used for the measurement of distance by a mekometer. Such an instrument can measure distance to an accuracy of 0.05 millimetres in 50 metres.

The magneto-optical effect occurs when plane-polarized light is reflected from a highly polished pole face of a ferromagnet. Slight elliptical polarization of the light beam is produced.