Question

Why do we take take the focal length of concave lens as negative and that of convex as positive?

Answer 1

in case of concavemirror then image is negative this shows focal length is even negative as per the lens formulae
the power is a reciprocal of focal length of lens
thus power is even negative


but in case of convex mirror the the image distance is positive as a result power event positive since focal length is even positive

Answer 2

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