Question

define. focal length of spherical mirror

Answer 1

Focal length of a spherical mirror is distance between the focus and the pole. 

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Answer 2

Focal length of the spherical mirror is defined as the distance of pole of mirror from the point on the axis of the mirror at which the rays, parallel to the principle axis, appear to converge. In other words, distance between the pole of mirror and the focus point called as the focal length of mirror