Q23 of 56 Page 1

(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.

(a) Power is defined as the reciprocal of its focal length and is a measure of the degree of convergence or divergence of light rays falling on the lens.

(b) The S.I. unit of power of a lens is Diopter.


1 diopter is the power of the lens whose focal length is 1m.


The S.I. unit of the power of the lens is diopter.


(c) Given: focal length of convex lens, f1=25 cm


In meters, f1 =0.25 m


Focal length of concave lens f2 = -10 cm


In meters, f2 =-0.10 m


Since power of a lens is the reciprocal of the focal length


P = 1/f




Power of combination is given by


P = P1+P2


P = 4 + (-10)


P= -6D


Thus, the power of the lens is 6D and it is concave lens because its power is negative.


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