Concave Mirror

Concave spherical mirrors reflect light with exactly the same reflection law as plane mirrors. However, since the surface is curved, special considerations apply. In particular we can find three special rays:

Any ray that is incident through the center of curvature, C, will hit the surface of the mirror perpendicularly and is thus reflected onto itself.
Any ray incident parallel to the axis of the mirror is reflected through the focal point, F. (The axis is the black horizontal line. By definition, the focal point lies on the axis.)
Of course this also implies that any ray incident through the focal point will be reflected parallel to the axis.

The distance between the center of curvature and the mirror surface is called radius, r. The distance between the focal point and the mirror surface (along the axis) is called focal length, f. One can show by geometry that:

r = 2 f.