Question

Two thin lenses are of same focal length (f), but one is convex and the other is concave. When they are placed in opposite with each other, the equivalent focal of the combination will be:

• A. $\frac{f}{4}$
• B. $\frac{f}{2}$
• C. Infinite
• D. Zero

Answer 1

Answer: (C)

Infinite

Answer 2

$\frac{1}{f_{eq}}=\frac{1}{f_1}+\frac{1}{f_2}$

$\frac{1}{f_{eq}}=\frac{1}{f}-\frac{1}{f}$

$f_{eq}=\infty$

So, the correct option is (C): Infinite.