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The refractive index of the material of a prism is √2 and its refracting angle is 30°. One of the refracting surfaces of the prism is made a mirror inwards. A beam of monochromatic light enters the prism from the mirror surface if its angle of incidence of the prism is

1. 30°
2. 45°
3. 60°
4. 0°

Correct answer: 45°

Solution

For total internal reflection to occur at the mirrored surface, the angle of incidence must be equal to or greater than the critical angle. Using the given refractive index (√2), the critical angle is 45°. Hence, the angle of incidence of the prism must be 45°.

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