Q. A light ray from air is incident (as shown in figure) at one end of a glass fibre (refractive index $\mu = 1.5$) making an incidence angle of $60{^\circ}$ on the lateral surface, so that it undergoes a total internal reflection. How much time would it take to traverse the straight fibre of length 1 $km$
A) 3.33 $\mu\ $s
B) 6.67 $\mu$ s
C) 5.77 $\mu$ s
D) 3.85 $\mu$ s
Explanation
(d) When total internal reflection just takes place from lateral surface $i = C,$ i.$e.,60{^\circ} = C$
$\Rightarrow \sin{60{^\circ}} = \sin C = \frac{1}{\mu} \Rightarrow \mu = \frac{2}{\sqrt{3}}$
Time taken by light to traverse some distance in a medium $t = \frac{\mu x}{c} = \frac{\frac{2}{\sqrt{3}} \times 10^{3}}{3 \times 10^{8}}3.85\ \mu\ s.$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.