<P> For a spherically curved mirror in air, the magnitude of the focal length is equal to the radius of curvature of the mirror divided by two . The focal length is positive for a concave mirror, and negative for a convex mirror . In the sign convention used in optical design, a concave mirror has negative radius of curvature, so </P> <Dl> <Dd> f = − R 2, (\ displaystyle f = - (R \ over 2),) </Dd> </Dl> <Dd> f = − R 2, (\ displaystyle f = - (R \ over 2),) </Dd> <P> where R is the radius of curvature of the mirror's surface . </P>

Focal length of convex lens is always positive