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.

Explanation

Power of lens = Ability to converge / diverge light rays passing through it reciprocal of the focal length in metres/ `1/f` (in metres)
SI unit power is Dioptre
Power of 1st lens `P_1 =(100)/(f_1)=(100)/(40 cm)` = + 2.5D
Nature: Converging lens/Convex lens
Power of 2nd lens `P_2 = (100)/(f_2)=(100)/(-20cm)` - 5D
Nature: Diverging lens/Concave lens