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In Young's double-slit experiment, 60 fringes are seen in the field of view when light of wavelength 4000 angstrom is used. How many fringes will appear in the same field of view if the wavelength is changed to 6000 angstrom?

1. 40
2. 90
3. 60
4. 50

Correct answer: 40

Solution

Fringe width beta = lambda*D/d, so the number of fringes in a fixed field of view N = W/beta is inversely proportional to lambda. Thus N1*lambda1 = N2*lambda2. So N2 = N1*lambda1/lambda2 = 60*4000/6000 = 40.

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