Light travels through a glass plate of thickness $t$ and having a refractive index $\mu$. If $c$ is the velocity of light in vacuum, the time taken by light to travel this thickness of glass is

The correct option is B $\frac{\mu t}{c}$
Let $v$ be the speed of light in glass.
Time taken by light to travel the glass thickness is,
time $=\frac{t}{\upsilon }$ .......(1)
from definition of refractive index,
$\mu =\frac{c}{\upsilon }$ .......(2)
From Eq. (1) & (2),
Time $=\frac{t}{\left(\frac{c}{\mu }\right)}$
$\therefore$ Time $=\frac{\mu t}{c}$
Hence, option $\left(b\right)$ is the correct answer.

Why this question? To give you an example of a low optical denser medium retard the speed of light. In this case $\frac{\mu t}{c}>\frac{t}{c},$ hence time taken to cross glass increased.