Question

A convex lens $A$ of focal length $20\, cm$ and a concave lens $B$ of focal length $56\, cm$ are kept along the same axis with the distance $d$ between them. If a parallel beam of light falling on $A$ leaves $B$ as a parallel beam, beam then distance, $d$ in $cm$, will be

• A. 25
• B. 36
• C. 30
• D. 50

Answer 1

Answer: (B)

As, power of combination, $P=\frac{1}{f_{1}}+\frac{1}{f_{2}}-\frac{d}{f_{1} f_{2}}=0$
$\therefore \frac{1}{f_{1}}+\frac{1}{f_{2}}=\frac{d}{f_{1} f_{2}}$
$\Rightarrow \frac{1}{20}-\frac{1}{56}=\frac{d}{20(-56)}$
$\Rightarrow \frac{56-20}{20 \times 56}=\frac{d}{-20 \times 56}$
$d=-36 \,cm$