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In Young’s double-slit experiment, if the slit separation is reduced to half its original value and the screen is moved to twice its original distance from the slits, what happens to the fringe width?

1. It becomes half of the original value
2. It becomes twice the original value
3. It becomes four times the original value
4. It does not change

Correct answer: It becomes four times the original value

Solution

Fringe width beta = lambda*D/d. With d -> d/2 and D -> 2D, beta -> lambda(2D)/(d/2) = 4*lambda*D/d, i.e. four times the original.

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