Spherical Mirror Focal Length. The mirror formula explains how object distance (u) and image distance (v) are related to the focal length of a spherical mirror. The point which lies midway between the vertex and the centre of curvature is the focal point, marked as ‘f’ in the figure.

The point which lies midway between the vertex and the centre of curvature is the focal point, marked as ‘f’ in the figure. There are two kinds of spherical mirrors, concave and convex.