A thin lens made of glass (refractive index =1.5)


A thin lens made of glass (refractive index $=1.5$ ) of focal length $f=16 \mathrm{~cm}$ is immersed in a liquid of refractive index 1.42. If its focal length in liquid is $f_{l}$, then the ratio $f_{l}$ /f is closest to the integer:

  1. 1

  2. 9

  3. 5

  4. 17

Correct Option: 2


(2) Using lens maker's formula


Here, $\mu_{g}$ and $\mu_{a}$ are the refractive index of glass and air respectively                   ....(i)

$\Rightarrow \frac{1}{f}=(1.5-1)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$

When immersed in liquid


Dividing (i) by (ii)

$\Rightarrow \quad \frac{f_{l}}{f}=\frac{(1.5-1) 1.42}{0.08}=\frac{1.42}{0.16}=\frac{142}{16} \approx 9$

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