Question

why is the focal length of a concave mirror taken as positive​

Answer 1

focal length of a concave mirror is negative.

focal length of a convex mirror is positive because of sign convention.

sign convention rules:

• all the values that go in direction of light i.e. to the right of the mirror are taken as positive and to the left or against the direction of light is taken as negative

as focus is on the right to the convex mirror, its length is taken as positive.

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