Certain materials change their optical properties when subjected to an electric field. This is caused by forces that distort the positions, orientations, or shapes of the molecules constituting the material. The electro-optic effect is the change in the refractive index resulting from the application of a dc or low-frequency electric field (Fig. 18.0-l). A field applied to an anisotropic electro-optic material modifies its refractive indices and thereby its effect on polarized light.