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In a Young's double-slit experiment the source, initially symmetric with respect to the two slits, is shifted parallel to the plane of the slits so that it lies closer to the upper slit. What happens to the fringe pattern?

1. The fringe width stays the same but the pattern shifts down.
2. The fringe width increases and the pattern shifts down.
3. The fringe width stays the same but the pattern shifts up.
4. The fringe width decreases and the pattern shifts down.

Correct answer: The fringe width stays the same but the pattern shifts down.

Solution

Fringe width beta = lambda*D/d is independent of where the source is. Moving the source up makes the path from source to upper slit longer; the central (zero path-difference) fringe moves to compensate, in the direction opposite to the source shift, i.e. downward.

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