A light ray going from air is incident (as shown in figure) at one end of a optical fibre used for communication purpose (refractive index of core `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 fibre of length 1 km?

Options

A. `4.25xx10^(-5)s`
B. `3.85xx10^(-6)s`
C. `5.77xx10^(-6)s`
D. `4.85xx10^(-5)s`

Answer

C

Explanation

When total internal reflection just takes place form lateral surface
`i=c, i.e., 60^(@)C=c`
`implies60^(@)=sinc=(1)/(mu)`
`impliesmu=(2)/(sqrt(3))`
time taken by light to traverse some distance in a medium
`t=(muxxl)/(c)=((2)/(sqrt(3))xx10^(3))/(3xx10^(8))=3.85mus`