Chapter-9|Ray Optics and Optical Instrument|NCERT 12th Physics:
Focal Length of Spherical Mirrors:
We consider a ray AB parallel to principal axis, incident at point B of a spherical mirror(concave or convex) of small aperture. After reflection from the mirror, this ray converges to point F(in case of concave mirror) or appears to diverge from point F(in case of convex mirror) obeying the law of reflection.
Thus F is the focus of the mirror, C is the centre of curvature, CP= the radius of curvature and BC is a normal to the mirror at point B.
