Question

A microscope is focused 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. 1 cm upward
• B. 4.5 cm downward
• C. 1 cm downward
• D. 2 cm upward

Answer 1

Answer: (A)

1 cm upward

Answer 2

Apparent depth of mark as seen through a glass slab of thickness $x$ and refractive index $\mu$ , is Apparent depth $=\frac{\text{Real depth}}{\text{Refractive index}}$ or $x=\frac{x}{\mu }=\frac{3}{1.5}=2 \,cm$ As image appears to be raised by $1 \,cm$, therefore, microscope must be moved upward by $1 \,cm$