Coherence required for interference

Steps:

• Interference patterns form when waves from two sources superpose with stable phase relationship.
• Ordinary lamps emit light with random phases, lacking coherence.
• Incoherent light leads to fluctuating superposition, averaging to no visible pattern.
• Coherent sources like lasers produce stable fringes.

Why B is correct:

• Coherence ensures constant phase difference; incoherent light from lamps violates this, preventing sustained interference per wave optics principles.

Why the others are wrong:

• A: Extended sources can still interfere if coherent, as in Young's double-slit with slits as effective sources.
• C: Amplitude difference affects fringe contrast but not pattern formation; interference occurs regardless.
• D: White light interferes in thin films or short paths, producing colored fringes, not preventing patterns entirely.

Final answer: B