Question

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.

Answer 1

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

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