What is meant by power of a lens

Reflection & Refraction (C10)

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 lens is the ability to converge or diverge light rays passing through it. It is the reciprocal of the focal length in metres.

P = 1/𝑓

SI unit of power is Dioptre.

Power of 1st lens 𝑃1 = 100/𝑓1 = 100/40 cm = +2.5 D

Nature: Converging lens / Convex lens

Power of 2nd lens 𝑃2 = 100/𝑓2 = 100/−20 cm = -5 D

Nature: Diverging lens / Concave lens