The relationship between the radius of curvature (R ) and the focal length (f) of a spherical mirror is:

• When a beam of parallel rays is incident on a concave mirror, rays meet at focus after reflection and the distance between the pole and the focus is called the focal length.
• The radius of the mirror is the radius of the sphere of which the spherical mirror is a part. 
• The relationship between focal length and aperture ($\mathrm{f}$) and the radius of curvature ($\mathrm{R}$):

         $\mathrm{f}=\frac{\mathrm{R}}{2} \mathrm{Or} \mathrm{R}=2\mathrm{f}$