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A microscope is focused on an ink mark on the top of a table. If we place a glass slab 3 cm thick on it, how should the microscope be moved to focus the ink spot again? The refractive index of glass is 1.5.

Asked in GUJCET 2011 · Snell's law, slabs and apparent depth

• (1) 2 cm upwards
• (2) 2 cm downwards
• (3) 1 cm upwards
• (4) 1 cm downwards

Step-by-step solution

Why the other options are wrong

• (1) 2 cm is the apparent thickness of the slab, t/n=3/(1.5). The shift is the difference between real and apparent thickness, 3-2=1 cm.
• (2) Wrong size and wrong direction: 2 cm is the apparent thickness t/n, and the mark appears raised, so the microscope must go up.
• (4) The size is right but the direction is not. Looking through the slab makes the mark appear nearer the eye, so the microscope has to be raised to follow it.

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