When a beam of polarised light reflects off a magnetised surface, the plane of polarisation of the light can slightly rotate. This phenomenon is known as the magneto-optic Kerr effect, named after Reverend Kerr who discovered the effect in the 19th Century. The technique is sometimes referred to as SMOKE, where the S stands for surface. However, the light is known to penetrate about 20 nm into the surface for most metals which means that MOKE is not particularly surface sensitive.

The experimental setup normally involves passing laser light through a polarising filter and then reflecting the light off the sample. The light then passes through another cross-polarising filter. Slight changes in the plane of polarisation will thus cause variations in the detected light intensity after the second filter. MOKE is frequently used to measure the hysteresis loops of thin magnetic films, by studying the light intensity as a function of applied magnetic field.

MOKE can also be incorporated into microscopes so that magnetic domain imaging becomes possible. The more traditional optical microscopes can be used in this manner, or the more recent near-field microscopes (see NSOM).

MOKE can be combined with SHG to produce the technique SH-MOKE.