A lens is a transparent object with two refracting surfaces. One or both surfaces are curved in order to collect light from one direction, change the convergence of the light rays and focus them onto a surface in the other direction. Lenses are found in nature (e.g. the eye) and in technology (e.g. cameras).
Lenses can be used either to converge or to diverge light rays. A converging lens takes light and concentrates it into a small point (e.g. the eye). A diverging lens takes light from a small source and projects it outwards (e.g. a screen projector).
Most lenses have spherical surfaces; that is, the surface can be imagined as part of a large sphere. This is not the ideal shape for a lens but it is the cheapest and easiest to manufacture. A surface that curves outwards is called convex, a surface that curves inwards is called concave, and a flat surface is called planar.
The illustration below shows the range of lens types:
A camera lens collects light, concentrates and focuses it on a recording medium such as film or a digital CCD. Many lenses are actually multiple (compound) lenses, which means they have two or more lenses on the same axis.
The term lens is also used to describe other ways of changing the convergence of electromagnetic radiation; for example: Microwave lens, gravitational lens (used in astronomy).