A curved, ground, and polished piece of glass, moulded plastic, or other transparent material used for the refraction of light. A converging lens is one that brings the rays of a parallel beam of light to a real principal focus. They include biconvex, planoconvex, and converging meniscus lenses. Diverging lenses cause the rays of a parallel beam to diverge as if from a virtual principal focus; these include the biconcave, planoconcave, and diverging meniscus lenses.
The centre of curvature of a lens face is the centre of the sphere of which the surface of the lens is a part. The optical axis is the line joining the two centres of curvature of a lens or, in the case of a lens with one plane surface, the line through one centre of curvature that is normal to the plane surface. The optical centre of a lens is the point within a lens on the optical axis through which any rays entering the lens pass without deviation. The distance between the optical centre and the principal focus of a lens is called the focal length (f). The distance (v) between the lens and the image it forms is related to the distance (u) between the lens and the object by the lens equation:
1/v + 1/u = 1/f,
provided that the real-is-positive convention is used. This takes distances to real objects, images, and foci as positive; those to virtual objects, images, and foci as negative. The equation does not always apply if the alternative New Cartesian convention (see sign convention) is used.