The time taken by light to travel normally through a glass plate of thickness 1 mm would be: (Take refractive index of glass = 1.5)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( 5\times10^{-12}\,\mathrm{s} \)
Light travels more slowly in glass: its speed in a medium is \( v = c/\mu \). The time is the thickness divided by this speed.
\( \displaystyle v = \frac{c}{\mu} = \frac{3\times10^{8}}{1.5} = 2\times10^{8}\,\mathrm{m\,s^{-1}} \)
\( d = 1\,\mathrm{mm} = 1\times10^{-3}\,\mathrm{m} \)
\( \displaystyle t = \frac{d}{v} = \frac{1\times10^{-3}}{2\times10^{8}} = 5\times10^{-12}\,\mathrm{s} \)
The same result follows from the optical path \( \mu d = 1.5\,\mathrm{mm} \) travelled at speed \( c \).