Q. A convex lens of focal length $30\ cm$ and a concave lens of $10\ cm$ focal length are placed so as to have the same axis. If a parallel beam of light falling on convex lens leaves concave lens as a parallel beam, then the distance between two lenses will be
A) 40 $cm$
B) 30 $cm$
C) 20 $cm$
D) 10 $cm$
Explanation
(c) Let distance between lenses be $x$. As per the given condition, combination behaves as a plane glass plate, having focal length $\infty$
So by using $\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{x}{f_{1}f_{2}}$
$\Rightarrow \frac{1}{\infty} = \frac{1}{+ 30} + \frac{1}{- 10} - \frac{x}{( + 30)( - 10)} \Rightarrow x = 20\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.