Question

Two thin plano-convex lenses each of focal length $f$ are placed as shown in the figure. The ratio of their effective focal lengths in the three cases is

• A. $1:2:3$
• B. $1:2:1$
• C. $1:1:1$
• D. $3:2:1$

Answer 1

Answer: (C)

$1:1:1$

As the focal length of each plano-convex lens is $f$,

then the equivalent focal length of 2 thin plano-convex lenses place near to each other is given as:

$\frac{1}{F}=\frac{1}{f}+\frac{1}{f}$.

So all three combinations will have the same net focal length.

Hence the ratio of their effective focal length in the three cases will be $1:1:1$