Question

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

Answer 1

Power of lens is defined as the reciprocal of the focal length of the lens expressed in meter.

SI unit of power is Dioptre denoted by $D$.

Focal length of first lens $f_1=40cm=0.4m$

Since the focal length is positive, so it is a convex lens.

Power of lens $P_1=\frac{1}{f_1}=\frac{1}{0.4}=2.5D$

Focal length of second lens $f_2=-20cm=-0.2m$

Since the focal length is negative, so it is a concave lens.

Power of lens $P_2=\frac{1}{f_2}=\frac{1}{-0.2}=-5D$