Answer
$20\times10^{-12}s$ or $20ps$
Work Step by Step
refractive index is defined as
$n=\frac{c}{v}$
$c$=speed of light in vacuum
$v$= speed of light in material
we can rewrite above equation as
$v=\frac{c}{n}$
given that for glass window $n=1.5$
speed of light in vacuum $c=3\times10^8m/s$
speed of light in glass window will be $v=\frac{3\times10^8m/s}{1.5}=2\times10^8m/s$
$speed =\frac{distance}{time}$
or we can rewrite it to $time=\frac{distance}{speed}$
since light pass perpendicular to the glass window $distance=thickness=4.0\times10^{-3}m$
time taken will be
$time=\frac{4.0\times10^{-3}m}{2\times10^8m/s}$=$2\times10^{-11}s$=$20\times10^{-12}s$=$20ps$
