Question

Two similar plano convex lenses are combined together in three different combinations as shown in figure. The ratio of the focal lengths in three cases 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

Let focal length of one plano convex lens$=f$

Then in all three cases their combined design will have focal length equal to

$\frac{1}{F}=\frac{1}{f}+\frac{1}{f}=\frac{2}{f}\therefore F=\frac{f}{2}$

Hence ratio $1:1:1$