The electro-optic effect is a result of the change in refractive index of a material when it is placed in DC or low-frequency electric field. Nonlinear optical materials with an incident static or low frequency electromagnetic wave will undergo modulation of refractive index and as such are often used as electro-optic modulators (EOM). In general, the modulation in refractive index in EOM may be induced on the amplitude, phase, frequency or polarization of the input beam. The most common EOM consists of a crystal whose refractive index depends on the strength of applied electric field. This implies if it is exposed to electric field, light will travel slower through it than in air. The phase of the light leaving the crystal can be controlled by changing the incident electric field as the phase delay is directly proportional to the time it takes for light to pass through the crystal.