Question

A convex lens of focal length $20cm$ and a concave lens of focal length $f$ are mounted coaxially $5cm$ apart. Parallel beam of light incident on the convex lens emerges from the concave lens as a parallel beam. Then, $f$ in cm is

• A. $35$
• B. $25$
• C. $20$
• D. $15$

Answer 1

The correct option is D $15$

$\frac{1}{f_{final}}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{x}{f_1{f}_2}$

as the rays from infinity travel undeviated $f_{final}$ is $\infty$

$0=\frac{1}{f_1}+\frac{1}{f_2}-\frac{x}{f_1{f}_2}$

$0=\frac{1}{20}+\frac{1}{f}-\frac{5}{20f}$

$f=-15$

option $D$ is correct