The Scottish physicist John Kerr (1824–1907) discovered the magneto-optical effect named after him in 1888. When linearly polarized light is reflected from the polished surface of a magnetized medium its polarization vector rotates and becomes somewhat elliptical. The direction of rotation and the sense of ellipticity are reversed when the direction of magnetization M of the sample is reversed, thus providing a powerful tool for optically monitoring the state of magnetization of the sample under investigation.

The physical mechanism of the Kerr effect is identical to that of the Faraday effect and, in fact, the same theoretical model can be used to describe both phenomena, one in reflection, the other in transmission (see Chapter 12, “The Faraday effect”).

The Kerr effect can be analyzed under quite general conditions, with the direction of magnetization of the sample oriented arbitrarily relative to the plane of incidence of the light beam. However, the three geometries shown in Figure 13.1 are of particular importance and will be analyzed separately in the present chapter. When the magnetization M is perpendicular to the sample's surface, the observed phenomenon is referred to as the polar Kerr effect. When M is parallel to the surface and in the plane of incidence, the Kerr effect is longitudinal. Finally, when M is parallel to the surface but perpendicular to the plane of incidence, the observed phenomenon is known as the transverse Kerr effect.