Question

(a) What is meant by ‘power of a lens’? 

(b) State and define the S.I. unit of power of a lens. 

(c) A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with each other. Calculate the lens power of this combination. 

Answer 1

(a) Power of a Lens: The ability of a lens, to converge or diverge the ray of light after refraction, is called power (P) of the lens. It is defined as the reciprocal of the focal length, i.e. 

p = 1/f

(b) The SI unit of power of a lens is ‘dioptre’. A lens of focal length 100 cm has a power of 1 dioptre, i.e. 1 dioptre = 1m^–1.

(c) Power of the combination p = p₁ + p₂ = (100/25 +100/-10) = 4-10 = -60