The area of the mirror from which reflection takes place is called its aperture. The length of the straight line joining two opposite extreme points on the periphery of a mirror is called linear aperture, and th e angle subtended by the two opposite extreme points of the periphery of a mirror at the centre of curvature is called the angular aperture. Note: The aperture basically denotes the size of the mirror.

The distance between the pole and the principal focus is called the focal length of the spherical mirror. It is denoted by letter f . It is half the distance of the radius of the curvature R . Relation between radius of curvature and the focal length is f = R 2 .