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In Young's double-slit experiment, the slit separation d is varied while the wavelength and the screen distance are kept fixed. Identify the correct statement(s) about how the interference pattern changes.

1. The angular width does not change
2. The fringe width changes in inverse proportion
3. The positions of all maxima change
4. The positions of all minima change

Correct answer: The fringe width changes in inverse proportion

Solution

Fringe width beta = lambda*D/d, so with lambda and D fixed, beta is inversely proportional to d (option B correct). Angular fringe width = lambda/d, which depends on d, so it does change (option A wrong). The position of the n-th bright fringe is yₙ = n*lambda*D/d and the n-th dark fringe is y = (n+1/2)*lambda*D/d; both depend on d, so the positions of maxima and minima change as d changes (options C and D are also physically correct). The single best/primary answer that captures the quantitative behavior is that the fringe width changes in inverse proportion to d.

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