Two waves y₁ = A sin(kx − ωt) and y₂ = A sin(kx − ωt + φ) superpose. The resulting amplitude is

1. Superposition of two same-frequency waves with phase difference φ yields a resultant of the same frequency. 2. Amplitude of the resultant: A_res = 2 A cos(φ/2). 3. For φ = 0, constructive interference gives 2A; for φ = π destructive gives 0. 4. This is the basis of Young's double-slit interference. _Source: syllabus Physics, Ch 14 "Waves", §14.5_