Concave Mirrors

A is the center of the concave mirror, C is the center of the curvature, AC is called the principal axis of the mirror.

An incident ray (1) parallel to the principal axis is reflected at mirror surface and the reflected ray intersects the principal axis at F, the focal point at which parallel incident rays converge after being reflected. An incident ray (2) passing through the focus F is reflected so that it parallels the principal axis. An incident ray (3) passing through the center of curvature C is reflected back on its original path.