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Question

# A postage stamp kept below a rectangular glass block of refractive index 1.5 when viewed from vertically above, it appears to be raised by 7.0 mm. Calculate the thickness of the glass slab.

A

20 cm

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B

2.1 cm

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C

11 cm

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D

3 cm

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Solution

## The correct option is B 2.1 cm Refractive index of the glass = 1.5 Let the thickness of the glass block = x mm Stamp raised by = 7 mm Observed depth = (x - 7) mm Refractive index = Real DepthObserved Depth 32 = x(x−7) 3x - 21 = 2x x = 21 mm or 2.1 cm

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