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Two coherent point sources S1 and S2 are placed a small distance d apart along a line, and an interference pattern is observed on a screen placed perpendicular to the line joining the sources. The shape of the interference fringes obtained on this screen will be:

1. points
2. straight lines
3. concentric circles
4. hyperbolas

Correct answer: concentric circles

Solution

The set of points with constant path difference from two point sources forms hyperboloids of revolution whose axis is the line through S1 and S2. When the screen is placed perpendicular to this axis, it intersects these surfaces in circles centred on the axis. Hence the fringes are concentric circles.

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