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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?

1. 4.5 cm downward
2. 1 cm downward
3. 2 cm upward
4. 1 cm upward

Correct answer: 1 cm upward

Solution

When a glass slab of thickness t and refractive index μ is placed, the apparent shift of the object is given by t(1 - 1/μ). Substituting t = 3 cm and μ = 1.5, the shift is 3(1 - 1/1.5) = 1 cm. The microscope must be moved 1 cm upward to refocus.

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