Question

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

• A. 25
• B. 15
• C. 50
• D. 30

Answer 1

Answer: (B)

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