The focal length,f=r/2

The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. A system with a shorter focal length bends the rays more sharply, bringing them to a focus in a shorter distance or diverging them more quickly. For the special case of a thin lens in air, a positive focal length is a distance over which initially collimated (parallel) rays are brought to a focus, or alternatively a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam.

• For spherical mirrors of small apertures, the radius of curvature is found to be equal to twice the focal length.
• The focal length (f) of a mirror is the distance between its pole (P) and principal focus (F).

• For spherical mirrors of small aperture, R = 2f.