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In a Young's double-slit setup, red light (7800 angstrom) and blue light (5200 angstrom) are used. Find the value of n for which the n-th red fringe coincides with the (n+1)-th blue fringe.

1. 2
2. 3
3. 4
4. 5

Correct answer: 2

Solution

Coincidence of the n-th red fringe with the (n+1)-th blue fringe requires equal distances from the central maximum: n*lambda_red = (n+1)*lambda_blue. Plugging in 7800 and 5200 angstrom and solving gives n = 2.

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