What is Lens ?

A lens is a transmissive optical device which affects the focusing of a light beam through refraction. A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements), usually along a common axis. Lenses are made from transparent materials such as glass, ground and polished to a desired shape. A lens can be used to focus light to form an image, unlike a prism which refracts light without focusing. Devices which similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses. ( http://en.wikipedia.org/wiki/Lens_(optics) ).