Optics Question 113
Question: 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
Options:
A) 40 cm
B) 30 cm
C) 20 cm
D) 10 cm
Show Answer
Answer:
Correct Answer: C
Solution:
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 $
