The center of this imaginary sphere is the mirror’s center of curvature, and the
radius of the sphere is called the mirror’s radius of curvature, R. Halfway
between the mirror and the center of curvature, C, is the focus (or focal point), F.
The intersection of the mirror’s optic axis (its axis of symmetry) with the mirror
itself is called the vertex, V, and the distance from V to F is called the focal
length, f, which is equal to one-half of the radius of curvature.
If the mirror had a parabolic cross-section, then any ray parallel to the axis would
be reflected by the mirror through the focal point. Spherical mirrors do this for
incident light rays near the axis (paraxial rays) because in the region of the mirror
that’s close to the axis, the shapes of a parabolic mirror and a spherical mirror are