Question

A light ray passes from air into another medium at point $P$. How long does it take the light ray to travel from $P$ to $Q$?

• A. $9\times {10}^{-10}s$
• B. $9\times {10}^{-7}s$
• C. $9\times {10}^{-8}s$
• D. $9\times {10}^{-6}s$

Answer 1

The correct option is C $9\times {10}^{-8}s$

from given ray diagram

$sinr=\frac{5}{12}$

$\mu =\frac{sini}{sinr}$

$\mu =\frac{sin{53}^0}{sinr}$

$\mu =\frac{4/5}{5/12}=\frac{48}{25}$

speed of light in medium is

$v_{medium}=\frac{c}{\mu }=\frac{3\times {10}^8}{48/25}=1.56\times {10}^{8}m/s$

the actual distance travel by light is

$d=\sqrt{5^2+{12}^2}=13m$

thus time taken by light ray to travel from P to Q

$t=\frac{d}{v_{medium}}=\frac{13m}{1.56\times {10}^{8}m/s}=8.33\times {10}^{-8}seconds\simeq 9\times {10}^{-8}seconds$

Option C is correct.