Question

A double convex lens of focal length $30cm$ is made of glass of R.I. $1.6$. When it is immersed in a liquid, the focal length is found to be $90cm$.The R.I. of the liquid is

• A. $1.33$
• B. $1.5$
• C. $1.4$
• D. $1.42$

Answer 1

The correct option is A $1.33$

$f=30cm\mu =1.6$

$\frac{f_1}{f_2}=\frac{({\mu }_2-1)}{(\mu -1)}$

$\frac{30}{90}=\frac{(\frac{1.6}{\mu }-1)}{(1.6-1)}$

$\frac{1}{3}\times 0.6=\frac{1.6-\mu }{\mu }$

$1.2\mu =1.6$

$\mu =4/3=1.33$