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Light enters at an angle of incidence in a transparent rod of refractive index n. For what value of the refractive index of the material of the rod the light once entered into it will not leave it through its lateral face whatsoever be the value of angle of incidence?

1. n > √2
2. n = 1
3. n = 1.1
4. n = 1.3

Correct answer: n > √2

Solution

For total internal reflection to occur at the lateral face for all angles of incidence, the critical angle must be less than or equal to 45°. This happens when the refractive index n > √2, as sin(critical angle) = 1/n and sin(45°) = 1/√2.

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