In optics a prism is a transparent optical element with smooth and polished surfaces that refract light. At least two of the flat surfaces must have an angle between them. The exact angles between the surfaces depend on the application. The traditional geometric shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial usage the "prism" generally refers to this type. Some types of optical prisms do not actually have the shape of geometric prisms. The prisms can be made of any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, plastic and fluorite.