The focal length of a spherical mirror is half its radius of curvature

The focal length of a spherical mirror is half its radius of curvature

In Fig. 22.3 AB is an incident ray parallel and close to the principal axis PC of a concave mirror. By definition, the reflected ray passes through the principal focus F. Since by geometry, the radius CB is normal to the mirror surface at B, it follows from the laws of reflection that,

The focal length of a spherical mirror is half its radius of curvature
The focal length of a spherical mirror is half its radius of curvature
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