The electro-optic effect refers to the change in the optical properties of a material when an electric field is applied. This phenomenon allows materials to alter their refractive index, which affects how light passes through them. It is commonly observed in certain crystals, such as LiNbO3 and KTP, and is utilized in various optical devices. This effect is crucial in technologies like modulators and switches, where controlling light signals is essential. By applying an electric field, these devices can manipulate light for applications in telecommunications and information processing, enhancing the performance of optical systems.