In figure, the optical fiber is \[\ell =2m\]long and has a diameter of \[d=20\mu m\]. If a ray of light is incident on one end of the fiber at angle \[{{\theta }_{1}}={{40}^{o}},\]the number of reflection it makes before emerging from the other end is close to: (refractive index of fibre is 1.31 and sin \[40{}^\circ \text{ }=\text{ }0.64\]) [JEE Main 8-4-2019 Morning] |
A) 55000
B) 57000
C) 66000
D) 45000
Correct Answer: B
Solution :
[b] Note : If we approximate the angle as initially then answer will be closer to 57000. But if we solve thoroughly, answer will be close to 55000. |
So both the answers must be awarded. |
Detailed solution is as following. |
Exact solution |
by Snells' law 1. |
Now |
Now no. of reflections |
Approximate solution |
By Snells' law 1. |
If assume |
Now number of reflections |
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