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For two-source interference to produce a stable observable fringe pattern, the sources must be:
AOf the same intensity only
BCoherent — same frequency with a constant phase difference
COf the same wavelength but arbitrary phase
DIncoherent but in phase initially
Answer & Solution
Correct answer: B. Coherent — same frequency with a constant phase difference
A stable fringe pattern requires that the phase difference between the two interfering waves at any point be constant in time. This requires both the same frequency (so the relative phase does not evolve) and a fixed initial phase relationship, which is the definition of coherent sources. Equal amplitudes only give maximum contrast; they are not strictly necessary.
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