70. 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 ?

70. 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 ?

(a) 4.5 cm downward

(b) 1 cm downward

(c) 2 cm upward

(d) 1 cm upward

Answer :  (d) 1 cm upward

\(AD = \frac{RD}{\mu}\)

\(AS = RD - \frac{RD}{\mu}\)

\(= RD \left(1 - \frac{1}{\mu}\right)\)

\(= 3 \left(1 - \frac{1}{1.5}\right)\)

\(= 1 \, \text{cm}\)