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For two light waves to produce a sustained interference pattern, which of the following is NOT a necessary condition?

1. the same frequency
2. the same amplitude
3. a constant phase difference
4. the same wavelength

Correct answer: the same amplitude

Solution

Sustained (stable) interference requires the sources to be coherent: same frequency (hence same wavelength in the medium) and a constant phase difference. Equal amplitude is not necessary - it only affects the fringe contrast (perfect dark fringes need equal amplitudes), but interference still occurs with unequal amplitudes. So the same amplitude is NOT a necessary condition.

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