Coherence in Double-Slit Interference

Steps:

• Define coherence: Sources are coherent if waves maintain a fixed phase relationship over time.
• In double-slit setup, single-frequency light ensures waves from slits have stable phase difference.
• Constant phase difference allows predictable interference patterns, unlike incoherent sources with random phases.
• Identify option matching this: B describes the essential condition for sustained interference.

Why B is correct:

• Coherence requires a constant phase difference between waves, as per the definition in wave optics, enabling stable maxima and minima in the interference pattern.

Why the others are wrong:

• A: Equal amplitude affects intensity but not phase stability needed for coherence.
• C: Constructive interference occurs only at specific path differences, not everywhere, regardless of coherence.
• D: Destructive interference also happens selectively, not universally, and isn't the definition of coherence.

Final answer: B