A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. It is a piece of transparent material, usually circular in shape, with two polished surfaces, either or both of which is curved and may be either convex (bulging) or concave (depressed) . Lenses are made from materials such as glass or plastic and are ground, polished, or molded to the required shape. They can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly focus or disperse waves and radiation other than visible light are also called "lenses", such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses.
Lenses are used in various imaging devices such as telescopes, binoculars, and cameras. They are also used as visual aids in glasses to correct defects of vision such as myopia and hypermetropia. In cameras, not one but a group of lenses are used for the gathering of light. The primary function of the lens of the eye is to bend and focus light to create a sharp image. To do that, the lens uses the help of ciliary muscles.
