Previous Year NEET Question- Wave Optics L-3

Question: Time taken by sunlight to pass through a window of thickness $4 \mathrm{~mm}$ whose refractive index is $\frac{3}{2}$ is#

A) $2 \times 10^{-4} \mathrm{~s}$

B) $2 \times 10^8 \mathrm{~s}$

C) $2 \times 10^{-11} \mathrm{~s}$

D) $2 \times 10^{11} \mathrm{~s}$

Answer: $2 \times 10^{-11} \mathrm{~s}$#

Sol:#

$\begin{aligned} & v_g=\frac{c}{\mu}=\frac{3 \times 10^8}{\frac{3}{2}}=2 \times 10^8 \mathrm{~m} / \mathrm{s} \ & t=\frac{x}{v_g}=\frac{4 \times 10^{-3}}{2 \times 10^8}=2 \times 10^{-11} \mathrm{~s} \end{aligned}$