First slide

Total internal reflection - Ray optics and optical instruments

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 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

Moderate

A

3.33 $μ$ sec
B

6.67 $μ$ sec
C

5.77 $μ$ sec
D

3.85 $μ$ sec

Solution

When total internal reflection just takes place from lateral surface $i = C i.e., 60^{\circ} = C$
Time taken by light to traverse some distance in a medium $t = \frac{μx}{c} = \frac{\frac{2}{\sqrt{3}} \times 10^{3}}{3 \times 10^{8}} = 3 .85 μsec$

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