A postage stamp kept below a rectangular glass slab 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.[3 marks]

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Solution

Let the thickness of the glass block be x mm

According to the question, apparent depth of the postage stamp = (x - 7) mm

We know that the refractive index of a glass is expressed as the following.

Refractive index of the glass = $\frac{Real thickness of the glass block}{Apparent thickness of the glass block}$

$1.5 = \frac{x}{x - 7} \Rightarrow 1.5 x - 1.5 \times 7 = x \Rightarrow 1.5 x - x = 10.5 \Rightarrow 0.5 x = 10.5 \Rightarrow x = \frac{10.5}{0.5} = 21 m m = 2.1 c m$

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