A plane progressive sound wave is travelling along the positive x–direction in air. Answer the following questions with suitable reasoning.
(i) Define the phase difference between two points on a wave.
(ii) Obtain an expression that relates the phase difference between two points separated by a distance Δx along the direction of propagation, in terms of the wavelength λ of the wave.
(iii) Hence state the conditions for the two points to oscillate (a) in phase, (b) in opposite phase.