Q.

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?

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a

3.33 µ sec

b

6.67 µ sec

c

3.85 µ sec

d

5.77 µ sec

answer is C.

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Detailed Solution

When total internal reflection just takes place from lateral surf'ace i = Critical angle = 600

Time taken by light to traverse length x in the medium = xv=xcμ=μxc

If L be the length of the fibre then x = LSin 600=2L3

Therefore time t = 2μL3c=2×1.5×10003×3×108second=5.77 μs

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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?