Optics Question 601
Question: A double convex thin lens made of glass (refractive index $ \mu $ = 1.5) has both radii of curvature of magnitude 20 cm. Incident light rays parallel to the axis of the lens will converge at a distance L such that
[MNR 1991; MP PET 1996; UPSEAT 2000; Pb PET 2004]
Options:
A) L = 20 cm
B) L = 10 cm
C) L = 40 cm
D) L = 20 / 3 cm
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Answer:
Correct Answer: A
Solution:
$ \frac{1}{f}=(\mu -1)( \frac{1}{R _{1}}-\frac{1}{R _{2}} ) $
Given $ R _{1}=+20cm,R _{2}=-20cm $ , $ \mu =1.5 $
$ \Rightarrow $ $ f=20 $ $ cm $ . Parallel rays converge at focus. So L=f.
