Question

A convex lens $'A'$ of focal length $20\, cm$ and a concave lens $'B'$ of focal length $5\, cm$ are kept along the same axis with a distance $'d'$ between them. If a parallel beam of light falling on $'A'$ leaves $'B'$ as a parallel beam, then the distance $'d'$ in cm will be :

• A. 25
• B. 15
• C. 50
• D. 30

Answer 1

The Correct Option is B

We know that :
d = f₁ - f₂
Now, By using the formula
\(\frac{1}{f_1}+\frac{1}{f_1}-\frac{d}{f_1f_2}=\frac{1}{F}\)
So, for the emergent beam to parallel :
F = a, P = 0 is given
Hence,
\(⇒\frac{1}{20}-\frac{1}{5}+\frac{d}{100}=0\)
\(⇒d=15\)
So, the correct option is (B) : 15.

Answer 2

Point \(F\) is the second focal length of \(A\) and first focal length of \(B\). 
\(\therefore d=(20-5) \,cm =15\, cm\)
So, the correct option is (B) : 15.