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In a Young's double-slit experiment, the screen distance D is varied (slit separation and wavelength fixed). Which statement is correct?

1. The angular fringe width does not change
2. The linear fringe width is independent of D
3. The change in fringe width is the same for all wavelengths
4. The position of the central maximum shifts

Correct answer: The angular fringe width does not change

Solution

The angular fringe width lambda/d depends only on wavelength and slit separation, so changing D leaves it unchanged. The linear fringe width lambda D/d is proportional to D (so it does change). The central maximum stays on the axis.

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