Two light sources can produce sustained interference fringes on a screen only if they are:
ACoherent (constant phase difference)
BOf identical intensity
CFrom the same physical lamp
DPolarised in the same direction
Answer & Solution
Correct answer: A. Coherent (constant phase difference)
Sustained interference needs sources with a constant phase relationship over time, which is what coherence means. Two ordinary lamps emit light with phases that vary randomly at $10^{-8}$ s scale, so their interference pattern washes out almost instantly. The two slits in Young's experiment achieve coherence because both are illuminated by the same primary source, inheriting its phase.
Equal intensity (B) improves fringe contrast but is not required. Same lamp (C) is one way to ensure coherence but not the only way (e.g. laser pairs). Polarisation alignment (D) matters for contrast, not for the existence of fringes.
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