Question

Two identical plano-convex lenses can be combined in three ways, as shown. The ratio of the focal lengths of these combinations will be

• A. 2 : 2 : 1
• B. 1 : 1 : 1
• C. 1 : 2 : 2
• D. 2 : 1 : 1

Answer 1

The correct option is B 1 : 1 : 1

In each case the equivalent focal length is given by
$\frac{1}{F}=\frac{1}{f}+\frac{1}{f}=\frac{2}{f}$
$F=\frac{f}{2}$
so,the ratio will be 1:1:1.
hence,option B is correct.