Question

A microscope is focussed on a mark on a piece of paper and then a slab of glass of thickness $3\text{cm}$ and refractive index $1.5$ is placed over the mark. How should the microscope be moved to get the mark in focus again?

• A. $1\text{cm}$ upward
• B. $4.5\text{cm}$ downward
• C. $1\text{cm}$ downward
• D. $2\text{cm}$ upward

Answer 1

The correct option is A $1\text{cm}$ upward

When glass slab is not present, the microscope was focussed on mark based on its actual depth 'd'


After placing a glass slab on mark '$\text{P}$', it will cause an upward shift in position of mark for the microscope.

We know that, Shift $S=t(1-\frac{1}{\mu })$

Substituting the given data we get,

$S=3(1-\frac{1}{1.5})$

or, $S=3(1-\frac{2}{3})=1\text{cm}$.

Thus in order to maintain focus on mark, the microscope has to be moved by 1 cm in upward direction.

Hence, option (a) is the correct answer.

Why this question? Tip: Here microscope has been given in question to derail your thought process. It's just a tool used by observer to view object. Bottomline: On placing glass slab the position of object appears shifted, thus to view it once again, you have to shift microscope by same amount.