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.

20 cm

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2.1 cm

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11 cm

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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 = $\frac{R e a l D e p t h}{O b s e r v e d D e p t h}$

$\frac{3}{2}$ = $\frac{x}{(x - 7)}$

3x - 21 = 2x

x = 21 mm or 2.1 cm

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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.

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