Two sources are said to be **coherent** if they:
AHave equal intensity
BEmit waves of the same frequency with a constant phase difference
CEmit waves of the same wavelength but random phase
DAre placed at the same point
Answer & Solution
Correct answer: B. Emit waves of the same frequency with a constant phase difference
Coherence requires identical frequency AND constant (time-independent) phase difference. This is why we cannot get a sustained interference pattern from two independent bulbs — we always use a single source split into two (Young's experiment).
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