Coherent sources have:

AConstant phase difference (and same frequency)

BDifferent frequencies

CDifferent wavelengths

DRandom phases

Answer & Solution

Correct answer: A. Constant phase difference (and same frequency)

**Coherent:** same frequency AND constant (typically zero) phase difference. Required for stable interference pattern. Lasers are highly coherent. Two independent bulbs are incoherent (no fixed phase relation).

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