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In a Young's double-slit experiment illuminated with white light, which statement is correct?

1. The fringe next to the central one will be red
2. The central fringe will be white
3. The fringe next to the central one will be violet
4. There will not be a completely dark fringe

Correct answer: The central fringe will be white

Solution

At the centre the path difference is zero for all wavelengths, so all colours add constructively and the central fringe is white. (The fringe immediately beside the centre is violet, since violet has the smallest wavelength and hence the smallest fringe width; this option, though also true in many texts, is secondary to the defining feature that the central maximum is white.)

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