Question

Question 37
Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm. What is the nature of the lens and its power used by each of them?

Answer 1

The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power. The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. The power P of a lens of focal length f (in m) is given by
$P=\frac{1}{f}$
The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.
The power of a convex lens is positive and that of a concave lens is negative. Thus, the lens with focal length 50 cm is convex and that with -50cm is convex. Their powers are +2 D and -2 D respectively.