Lens

[Sarah]

A piece of glass, or other transparent substance, ground withtwo opposite regular surfaces, either both curved, or one curved andthe other plane, and commonly used, either singly or combined, inoptical instruments, for changing the direction of rays of light, andthus magnifying objects, or otherwise modifying vision. In practice,the curved surfaces are usually spherical, though rarely cylindrical,or of some other figure. Lenses