Question

41. A light ray from air is incident (as shown in the figure at one end of a glass fibre u = 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?

a) 3.33 u sec
b) 6.67 u sec
c) 5.77 u sec
d) 3.85 u sec
​

Answer 1

Given :

Angle of incidence = 60°

Length = 1km

μ = 1.5

To Find :

Time taken for the light to traverse the straight fiber

Solution :

• As total internal reflection takes place from lateral surface, i=C

          Sin i = Sin C

          Sin 60° = 1/μ

          μ = 2/√3

• Time taken for the light to travel = μx /c

              =2/√3 × 10³ / 3×10⁸

              = 3.85 μsec

Time taken for the light to traverse the straight fiber is 3.85 μsec