define principal focus of a Convex lens and Concave lens

Principal focus of a convex lens is the point on the principal axis where rays incident parallel to the principal axis will pass through that point after getting refracted by the lens. In the fig. F denotes the principal focus of the lens.

Principal focus of a concave lens is the point on the principal axis where rays incident parallel to the principal axis will appear to diverge from that point when produced behind the lens, after getting refracted by the lens.  In the fig. F denotes the principal focus of the lens.

 

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is it the same as i mirrors?

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