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In Young’s double-slit experiment, if the yellow light from a sodium lamp is replaced by monochromatic blue light of the same intensity, what happens to the fringe width?

1. The fringe width decreases
2. The fringe width increases
3. The fringe width remains unchanged
4. The fringes become less intense

Correct answer: The fringe width decreases

Solution

Fringe width beta = lambda*D/d is proportional to wavelength. Blue light has a smaller wavelength than yellow, so the fringe width decreases.

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