Tutorial : focal lengths - principal planes - mirror

In the case of a mirror, principal planes coincide with the plane perpendicular to the optical axis and containing the mirror vertex O. The front and back focal points coincide. Therefore, focal lengths (f, f_i, f_t and f_b ) are then identical : f = f_i = f_t = f_b = R/2 where R is the mirror radius.

The mirror center C is a characteristic point as all light rays coming from C are reflected by the mirror backward in the same direction. The mirror vertex O is also characteristic as it reflects light rays in a symetrical direction regarding the optical axis.

Note that a plane mirror is an afocal system with a magnification of -1.