# 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.

### Answer

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