A microscope is focussed on an ink mark on the top of a table. If we place a glass slab of $3$cm thick on it, how should the microscope be moved to focus the ink spot again? The refractive index of glass is $1.5$.

Given,

The refractive index of the glass is $n=1.5$

The thickness of the glass slab is $h_o=3$cm

Refractive index is given as:

$n=\displaystyle\frac{h_o}{h_i}$

$1.5=\displaystyle\frac{3}{h_i}$

$\displaystyle h_i=\frac{3}{1.5}=2\ cm$

So, the microscope needs to be shifted upwards by:

$h'=3-2=1\ cm$ upwards.