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 each lens.

Power of a lens refers to the reciprocal or inverse of focal length measured in metre. It’s S.I unit is dioptre (D).

40 cm= 0.4m

And, -20 cm= -0.2m

P¹=1/f¹

P¹=1/0.4

P¹=2.5 D

And the lens will be convex as the focal length is in positive.

Now, in second case,

P²=1/f²

P²= 1/-0.2

P²=5 D

And here the lens will be concave as the focal length is in negative.