Ray Optics and Optical Instruments - Result Question 31
31. When a biconvex lens of glass having refractive index 1.47 is dipped in a liquid, it acts as a plane sheet of glass. This implies that the liquid must have refractive index.
[2012]
(a) equal to that of glass
(b) less then one
(c) greater than that of glass
(d) less then that of glass
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Answer:
Correct Answer: 31. (a)
Solution:
(a) $\frac{1}{f}=(\frac{\mu_g}{\mu_m}-1)(\frac{1}{R_1}-\frac{1}{R_2})$
If $\mu_g=\mu_m$, then $\frac{1}{f}=(1-1)(\frac{1}{R_1}-\frac{1}{R_2})$
$\Rightarrow \frac{1}{f}=0 \quad f=\frac{1}{0}=\infty$
This implies that the liquid must have refractive index equal to glass.
A transparent solid is invisible in a liquid of same refractive index because of no refraction.
