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A point source of light is placed 4 m below the surface of water of refractive index 5/3. The minimum diameter of a disc, which should be placed over the source, on the surface of water to cut off all light coming out of water is

1. ∞
2. 6 m
3. 4 m
4. 3 m

Correct answer: 3 m

Solution

The light rays from the source will undergo total internal reflection at the critical angle. The radius of the circle of light on the water surface is determined by the critical angle and the depth of the source. Using Snell's law and geometry, the minimum diameter of the disc is 3 m.

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