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In a Young's double-slit experiment using monochromatic light, how can the central bright fringe (zero-order maximum) be distinguished from the other bright fringes?

1. It has greater intensity than the other bright fringes.
2. It is wider than the other bright fringes.
3. It is narrower than the other bright fringes.
4. By using white light instead of monochromatic light, the central fringe appears white while others are coloured.

Correct answer: By using white light instead of monochromatic light, the central fringe appears white while others are coloured.

Solution

With single-wavelength light, every bright fringe has the same width and (ideally) the same intensity, so the central fringe cannot be picked out. If white light is used, only the central fringe (zero path difference) has all wavelengths coinciding and therefore appears white, while the surrounding fringes become coloured. Hence using white light identifies the central fringe.

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