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 a lens. The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it.
SI unit of power of lens is dioptre (D).
Given : `f = 40 cm = (40)/(100) m = 2/5 m`
`P = 1/(f(m)) = 1/(2//5) = 5/2 = + 2.5 D`
Nature of lens is convex lens.
Given : `f = - 20 cm = 20/1000 m = (-1)/(5) m`
`P = (1)/(f(m)) = (1)/(-1//5) = (-5)/(1) = -5 D`
Nature of lens is concave lens.