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Q.67 A green light is incident from the water to the air- water interface at the critical angle (θ). Select the correct statement -

1. The spectrum of visible light whose frequency is less than that of green light will come out to the air medium
2. The spectrum of visible light whose frequency is more than that of green light will come out to the air medium
3. The entire spectrum of visible light will come out of the water at various angles to the normal
4. The entire spectrum of visible light will come out of the water at an angle of 90° to the normal

Correct answer: The spectrum of visible light whose frequency is less than that of green light will come out to the air medium

Solution

Refractive index increases with frequency, so higher-frequency light has a smaller critical angle. Green is incident exactly at its own critical angle; colors of higher frequency (smaller critical angle) exceed their critical angle and are totally internally reflected, while colors of lower frequency (larger critical angle) are below theirs and refract out into the air.

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