Optics Question 92
Question: A hollow double concave lens is made of very thin transparent material. It can be filled with air or either of two liquids $L_1$ and $L_2$ having refractive indices $n_1$ and $n_2$ respectively ($n_2$>$n_1$>1). The lens will diverge a parallel beam of light if it is filled with
[IIT-JEE (Screening) 2000]
Options:
A) Air and placed in air
B) Air and immersed in $L_1$
C) $L_1$ and immersed in $L_2$
D) $L_2$ and immersed in $L_1$
Show Answer
Answer:
Correct Answer: D
Solution:
$ \frac{1}{f}=( \frac{n _{2}}{n _{1}}-1 )( \frac{1}{R _{1}}-\frac{1}{R _{2}} ) $ where $ n _{2} $ and $ n _{1} $ are the refractive indices of the material of the lens and of the surroundings respectively.
For a double concave lens, $ ( \frac{1}{R _{1}}-\frac{1}{R _{2}} ) $ is always negative.
Hence $ f $ is negative only when $ n _{2}>n _{1} $
BETA
