Question

A postage stamp appears raised by 7.00 mm, when placed under a glass block of refractive index 1.5. Find the thickness of the block.

Answer 1

Given: \[ \mu = 1.5 \]

Apparent rise:

\[ \text{Real depth} - \text{Apparent depth} = 7\,\text{mm} \]

We know,

\[ \mu=\frac{\text{Real depth}}{\text{Apparent depth}} \]

Let the thickness of the glass block be \(t\).

Then,

\[ \text{Apparent depth}=\frac{t}{1.5} \]

So,

\[ t-\frac{t}{1.5}=7 \]

\[ t-\frac{2t}{3}=7 \]

\[ \frac{t}{3}=7 \]

\[ t=21\,\text{mm} \]