The phenomenon that the Refractive Index of a medium exhibits a modification which is proportional to the strength of an applied electric field.

The Pockels effect (first described in 1906 by the German physicist Friedrich Pockels) is the linear electro-optic effect, where the Refractive Index of a medium is modified in proportion to the applied electric field strength. This effect can occur only in non-centrosymmetric materials.