Constructive Interference from Two Coherent Sources

Steps:

• Waves from two speakers interfere; maximum amplitude occurs where path difference is integer multiple of wavelength (constructive interference).
• Examine diagram: wavefronts show circular patterns from each speaker.
• For each point, compute path length difference to both speakers using wavefront spacing (one wavelength per interval).
• Point D lies on the perpendicular bisector, where paths are equal (difference = 0), so waves arrive in phase.

Why D is correct:

• Path difference of zero wavelengths satisfies constructive interference condition, doubling amplitude per superposition principle.

Why the others are wrong:

• A: Path difference ≈ λ/2, causing destructive interference and minimum sound.
• B: Path difference ≈ λ/4, partial destructive interference reduces amplitude.
• C: Path difference ≈ 3λ/2, destructive interference cancels waves.

Final answer: D