Two sources are said to be coherent if:
AThey have different frequencies
BThey emit waves with the same frequency AND a constant (time-independent) phase difference
CThey emit waves with the same amplitude only
DThey are placed close together
Answer & Solution
Correct answer: B. They emit waves with the same frequency AND a constant (time-independent) phase difference
**Coherence requires both: (i) same frequency, (ii) constant phase difference**. Two independent ordinary sources (like two sodium lamps) are NOT coherent because they have random phase fluctuations on a ~10⁻¹⁰ s timescale. Coherent sources are obtained by **dividing wavefronts** of a single source (YDSE) or by **laser sources**.
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