The time required for the light to pass through a glass slab (retractive Index=1.5) of thickness 4mm is $\left(c=3\times {10}^{8}m/s\right)$ 

Speed of light in glass slab is.Time required for the light to pass through a glass slab of thickness 4mm is

$\begin{array}{l}t=\frac{thickness}{velocity}=\frac{4mm}{3\times {10}^8/1.5}\\ =\frac{4\times {10}^{-3}}{3\times {10}^8/1.5}\\ t=2\times {10}^{-11}S\end{array}$