The change in the index of refraction of a material when subjected to an electric field.

The effect can be used to modulate a light beam in a material since many properties; such as, light-conducting velocities, reflection, and transmission coefficients at interfaces, acceptance angles, critical angles, and transmission modes, are dependent on the refractive indexes of the media in which the light travels.

The inducement of double refraction of light in a transparent substance when a strong electric field is applied in a direction transverse to the beam of light.

In double refraction, the index of refraction (a measure of the amount the ray is bent on entering the material), and hence the wave velocity of light vibrating in the direction of the electric field, is slightly different from the index of refraction of the vibration perpendicular to it.

Optically, the substance behaves like a crystal with its optic axis parallel to the electric field.

This effect was discovered in the latter part of the 19th century by a Scottish physicist, John Kerr.

The same behavior in solids is sometimes called the Pockels effect.