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A microscope is adjusted to focus on a spot marked on a sheet of paper. A glass slab 3 cm thick with refractive index 1.5 is then placed over the mark. In what direction and by how much must the microscope be shifted to bring the mark back into focus?

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

Correct answer: 1 cm upward

Solution

The slab raises the apparent position by t(1 - 1/n) = 3(1 - 1/1.5) = 3(1/3) = 1 cm. The microscope must be moved 1 cm upward to refocus.

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