First slide

Ray optics

Question

The focal length of a convex lens of refractive index 1.5 is 'f' when it is placed in air. When it is immersed in a liquid it behaves as a converging lens and its focal length becomes xf (x > 1). The refractive index of the liquid is

Difficult

A

> 3/2
B

< 3/2 and > 1
C

< 3/2
D

= 3/2

Solution

$\frac{f'}{f} = \frac{μ_{g} - 1}{(\frac{μ_{g}}{μ_{l}} - 1)}, f' = xf and x > 1 ∴ μ_{l} < μ_{g} \Rightarrow μ_{l} < \frac{3}{2}$
$μ_{l}$ is absolute refractive index always >1

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