A light ray from air is incident on a glass slab (μ = 1.5) as shown in the figure. It undergoes a total internal reflection and travels along AB. The thickness AC of the glass slab is 20 cm. The time taken by light to cross the glass slab after reflection is

2 ns

The correct answer is Option (2) → 2 ns

Given: $\mu=1.5,\;AC=0.20\ \text{m},\;\theta=60^\circ$ (angle with normal inside glass).

Path length inside glass after reflection: $s=\frac{AC}{\cos\theta}=\frac{0.20}{\cos60^\circ}=\frac{0.20}{0.5}=0.40\ \text{m}$

Speed of light in glass: $v=\frac{c}{\mu}=\frac{3\times10^8}{1.5}=2\times10^8\ \text{m/s}$

Time: $t=\frac{s}{v}=\frac{0.40}{2\times10^8}=2.0\times10^{-9}\ \text{s}=2\ \text{ns}$

Answer: $2\ \text{ns}$