Question

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

• A. $10^{-11} sec$
• B. $2\times10^{-11}\, sec$
• C. $2\times10^{+11} \,sec$
• D. $2\times10^{-5} \,sec$

Answer 1

Answer: (B)

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 $=\frac{\text { Distance }}{\text { Speed }}$
$t=\frac{4 \times 10^{-3}}{2 \times 10^{8}}$
Or $t=2 \times 10^{-11} s$