A
lens is a transmissive
optical device that affects the focus 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 focus light to form an
image, unlike a
prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as
microwave lenses or acoustic lenses.