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Two coherent sources of different intensities produce an interference pattern. The ratio of maximum intensity to minimum intensity is 25. Find the ratio of the individual intensities of the two sources.

1. 25: 1
2. 5: 1
3. 9: 4
4. 625: 1

Correct answer: 9: 4

Solution

Let r = sqrt(I1/I2). Then (r+1)²/(r-1)² = 25. So (r+1)/(r-1) = 5 -> r+1=5r-5 -> 4r=6 -> r=3/2. So I1/I2 = 9/4, i.e., ratio 9:4.

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