What is Principal Focus? - QS Study
QS Study

Principal focus: A beam of rays adjacent and parallel to the principal axis being incident on a spherical mirror converges at a point on the principal axis (in case of concave mirror) or appears to be diverging from a point on the principal axis (in case of convex mirror) after reflection, then this point is called the principal focus of the mirror.

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Focal length: The distance between the pole and the principal focus of a spherical mirror is called the focal length of the mirror. In figure (1) and (2), PF is the focal length. In a spherical mirror, the focal length is half of the radius of curvature. Therefore, f = r/2.

Focal plane: The plane imagined that passes through the principal focus and perpendicular to the principal axis is called the focal plane.