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 $



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Dual Pane