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# What do you mean by power of a lens?

Discussion in 'CBSE Class 10 Physics Help' started by Vinila Choudary, Jan 19, 2012.

1. ### Vinila ChoudaryActive Member

What do you mean by power of a lens? How is it related to the focal length of the lens? When do we say that power of a lens is positive or negative?

The power of a lens may be defined as the reciprocal of its focal length in metres. Its SI unit is dioptre (D) and it is denoted as P.

Power P = 1/ Focal length (f) in metres

= 1/f = 100/f(in cm)

This expression indicates that the power of a lens of shorter focal length is greater than the power of a lens of longer focal length. Power of a convex lens is positive and that of a concave lens is negative.