Concave and convex lenses can be further subdivided into many different types, including spherical lenses with circular surfaces, aspherical lenses with curved but aspherical surfaces, cylindrical lenses with an arched back, annular lenses shaped like doughnuts, and Fresnel lenses with washboard surfaces. On these lenses, light rays are refracted on the surface of the lens. However, there are other types of lenses, including self focusing lenses that produce optical effects with a stepwise refractive index (the stepwise refractive index is generated by the lens material, not by the refraction of the lens surface) and diffractive lenses that utilize the phenomenon of light wave dispersion. Self focusing lenses are commonly used in endoscopes, and diffractive lenses are commonly used in CD and DVD players.
