The definitions of the principal axis, centre of curvature , radius of curvature , and the vertex , of a convex mirror are analogous to the corresponding definitions for a concave mirror. When parallel light-rays strike a convex mirror they are reflected such that they appear to emanate from a single point located behind the mirror, as shown in Fig.This point is called the virtual focus of the mirror. The focal length of the mirror is simply the distance between and . As is easily demonstrated, in the paraxial approximation, the focal length of a convex mirror is half of its radius of curvature.