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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