Question

A light ray from air is incident (as shown in figure) at one end of a glass fiber (refractive index μ = 1.5) making an incidence angle of 60⁰ 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

• A. 3.33 μ sec
• B. 6.67 μ sec
• C. 5.77 μsec
• D. 3.85 μ sec

Answer 1

Answer: (D)

3.85 μ sec

Answer 2

When total internal reflection just takes place from lateral surface then i = C i.e. C = 60⁰

From $\mu = \frac { 1 } { \sin C }$ $\Rightarrow \mu = \frac { 1 } { \sin 60 } = \frac { 2 } { \sqrt { 3 } }$

Hence time taken by light traverse some distance in medium $t = \frac { \mu x } { C }$