Light - Reflection And Refraction

Question

What is meant by the power of a lens? Write its SI unit. A student uses a lens of focal length of 40 cm and another of –20 cm. Write the nature and power of each lens.

Solution

The power of lens is defined as the reciprocal of its length. It is denoted by using the letter P.

$P \propto \frac{1}{F}$

Unit of Power is Dioptre (D)

Focal length = 40 cm

Focal length = -20 cm

Convex lens P = 1/f = 100/40 = 2.5 D

Concave lens = P = 1/f = -100/20 = -5D