A light ray from air is incident (as shown in figure ) at one end of a glass fiber ( 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 traverse the straight fiber of length `1 km `?

Options

A. 3.3 `mu`s
B. 6.6 `mu`s
C. 5.7 `mu`s
D. 3.85 `mu`s

Answer

D

Explanation

(d) When total internal reflection just takes place from lateral surface, i=C i.e., `60^(@)=C`
`rArr sin 60^(@)=sinC= 1/murArrmu=2/sqrt3`
Time taken by light to traverse some distance in a medium,
`t=(mux)/c=(2/sqrt3xx10^(3))/(3xx10^(8))=3.85mus`