Question

The power of a lens with focal length 20cm is..

Answer 1

The power of a convex lens is positive and that of a concave lens is negative. Thus a convex lens of focal length 40 cm is said to possess a power of + 2.5 dioptre. similarly the power of a concave lens of focal length 20 cm is 5.0 dioptre.

Answer 2

it will 5.0 diopter
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