Optics Question 621

Question: The plane surface of a plano-convex lens of focal length f is silvered. It will behave as

[MP PMT/PET 1998]

Options:

A) Plane mirror

B) Convex mirror of focal length 2 $ f $

C) Concave mirror of focal length $ f/2 $

D) None of the above

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Answer:

Correct Answer: C

Solution:

Focal length of effective lens $ \frac{1}{F}=\frac{2}{f _{l}}+\frac{1}{f _{m}}=\frac{2}{f _{l}}+\frac{1}{\infty }\Rightarrow F=\frac{f _{l}}{2} $



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