Question

The time required for the light to pass through a glass slab (refractive index = 1.5) of thickness 4 mm is ($c=3\times {10}^{8}m{s}^{-1}$, speed of light in free space)

• A. ${10}^{-11}s$
• B. $2\times {10}^{-11}s$
• C. $2\times {10}^{11}s$
• D. $2\times {10}^{-5}s$

Answer 1

The correct option is B $2\times {10}^{-11}s$

We know, $n_a{c}_a=n_g{c}_g$
$\frac{n_g}{n_a}=\frac{c_a}{c_g}$
$\frac{3}{2}=\frac{3\times {10}^8}{c_g}$
$c_g=2\times {10}^8$
We have, $Time=\frac{Distance}{Speed}$
$t=\frac{4\times {10}^{-3}}{2\times {10}^8}$
$t=2\times {10}^{-11}s$.