Define focal length.

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Solution

The distance between the pole of the mirror ( or the optical center of the lens) and the focal point of a mirror (or lens) is called the focal length.
The focal point (F) of a mirror is the point at which a parallel beam of light is "focussed" after reflection in the mirror.
The focal length is taken as positive for a convex lens and convex mirror.
It is taken as negative for a concave lens and a concave mirror.
It is denoted by f. Here,
$T h u s P F = f$

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