What is meant by power of a lens? What does its sign (+ve or -ve) indicate? State its S.I. unit. How is this unit related to focal length of a lens?
CBSE DELHI - SET 3 2016
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The power of a lens may be defined as the reciprocal of its focal length in meters. Its SI unit is dioptre (D) and it is denoted as P. Power $\text{P}=\frac{1}{\text{Focal length (f) in meters}}$$=\frac{1}{\text{f}}=\frac{100}{\text{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.
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.
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