Ray Optics and Optical Instruments - Result Question 39
40. A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will
[2003]
(a) remain unchanged
(b) become zero
(c) become infinite
(d) become small, but non-zero
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Answer:
Correct Answer: 40. (c)
Solution:
(c) $\frac{1}{f}=( _{\ell} \mu_g-1)(\frac{1}{R_1}-\frac{1}{R_2})$
where, $\ell \mu_g=1$ is given.
$\Rightarrow \frac{1}{f}=(1-1)(\frac{1}{R_1}-\frac{1}{R_2})=0 \Rightarrow f=\infty$
