Question

A thin converging lens of power 6D and a thin diverging lens of power 2D are in contact. What is the focal length of the combination?

Answer 1

TOPIC :- LENS

Concept

• Power is related to focal length as

$p = \frac{1}{f(in \: m \: )} \: = \frac{100}{f(in \: cm)}$

• Power of a converging lens (convex lens) is positive since focal length is positive

• Power of a diverging (concave) lens is negative since focal length is negative.

• Equivalent focal length(f') of two lens is given having focal length f1 and f2 is given by

$\frac{1}{f} = \frac{1}{f1} + \frac{1}{f2}$

• Equivalent power of two lens is given as p' = P1 + P2.

SOLUTIONS

power of converging lens = 6 D

power of diverging lens = -2 D.

Equivalent power = 6 - 2 = 4 D.

Hence, focal length = 1/P = 1/4 = 0.25 m

Hence, focal length of the lens is 0.25 m.