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Two coherent monochromatic beams of intensities I and 4I are made to interfere. What are the maximum and minimum possible intensities in the resulting beam?

1. 5I and I
2. 5I and 3I
3. 9I and I
4. 9I and 3I

Correct answer: 9I and I

Solution

Amplitudes add or subtract: maximum when in phase gives (1+2)² I = 9I, minimum when out of phase gives (2-1)² I = I.

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