Question

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 on the lateral surface, so that it undergoes a total internal reflection. How much time would it take to reverse the straight fiber of length 1 km?

Answer 1

When total internal reflection just takes place from lateral surface $i=Ci.e.{60}^o=C$
$\Rightarrow sin{60}^o=sinC=\frac{1}{\mu }\Rightarrow \mu =\frac{2}{\sqrt{3}}$
Time taken by light to traverse some distance in a medium
$t=\frac{\mu x}{c}=\frac{\frac{2}{\sqrt{3}}\times {10}^3}{3\times {10}^8}=3.85\mu sec.$