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For two light sources to produce STABLE INTERFERENCE FRINGES, they must be:
AIdentical in colour but random phase relations
BCoherent (constant phase difference, same frequency)
CFrom two independent bulbs of any wattage rating
DPolarised in two perpendicular directions exactly
Answer & Solution
Correct answer: B. Coherent (constant phase difference, same frequency)
COHERENT sources = same frequency + constant phase difference. Required for stable interference. Independent ordinary bulbs are incoherent; can't produce sustained interference. Lasers are coherent.
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