Question

A travelling microscope is focussed on to a scratch on the bottom of a beaker. Water of refractive index is $\frac{4}{3}$ poured in it. Then the microscope is to be lifted through $2cm$ focus it again. Find the depth of the water in the beaker.

• A. $4cm$
• B. $8cm$
• C. $\frac{8}{7}cm$
• D. $\frac{8}{3}cm$

Answer 1

The correct option is B $8cm$

Apparent depth of bottom of beaker $d_{app}=\frac{n_{air}}{n_{water}}{d}_{real}$
Given, $d_{app}=d_{real}-2$
$\therefore$ $d_{real}-2=\frac{3}{4}{d}_{real}$
$⟹d_{real}=8cm$