The distance between the principal focus of a mirror and the pole of mirror is called focal length of the mirror.
In case of Concave mirror distance between the pole ‘O’ and principal focus ‘F’ of mirror is focal length and is always taken as negative.
In case of Convex mirror distance between the pole ‘O’ and principal focus ‘F’ of mirror is focal length and is always taken as positive.
The radius of Curvature of plane mirror is infinite, hence the focal length of plane mirror will also be infinite.
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