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In a Young’s double-slit setup, two monochromatic sources emit light of wavelengths 2500 Å and 3500 Å. Which pair of fringe orders from the two patterns will fall at the same position?

1. 3rd order of the first source and 5th order of the second
2. 7th order of the first source and 5th order of the second
3. 5th order of the first source and 3rd order of the second
4. 5th order of the first source and 7th order of the second

Correct answer: 7th order of the first source and 5th order of the second

Solution

Fringes coincide when n1*2500 = n2*3500, giving n1/n2 = 3500/2500 = 7/5. So the 7th order of the 2500 A source overlaps the 5th order of the 3500 A source.

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