question_answer

A microscope is focussed on a mark on a piece of paper and then a slab of glass of thickness 3 cm and refractive index 1.5 is placed over the mark. How should the microscope be moved to get the mark in focus again?            [AIPMT (S) 2006]

A) 1 cm upward

B)                   4.5 cm downward

C) 1 cm downward

D)      2 cm upward

Correct Answer: D

Solution :

Apparent depth of mark as seen through a glass slab of thickness x and refractive index  is

Apparent depth

or

As image appears to be raised by 1 cm, therefore, microscope must be moved upward by 1 cm.