Abstract: Switchable optical devices are made of transparent optical elements having surfaces separated by small distances, typically less than a few hundred micrometers. The transparent elements contain roughness, kinoforms, Fresnel-lens, or other relief structures with predetermined optical properties that affect the passage of light through the optical elements by refraction and/or diffraction. By introducing a liquid with a refractive index matching that of the transparent elements' material into the space between the surfaces, the effects of the surface structures disappear almost completely, thereby changing the optical characteristics of the device. This effect is used advantageously to change the focal properties, scattering properties, or light directing properties of different kinds of optical devices.