# Time taken by sunlight to pass through a window of thickness 4 mm whose refractive index is $\large\frac{3}{2}$ is
$\begin{array}{1 1} 2 \times 10^{-4} \;s \\ 2 \times 10^8 \;s \\ 2 \times 10^{-11}s \\ 2 \times 10^{11} s \end{array}$