Question: how to calculate path difference between two waves?
Answer: Observe that the wave from Source 1 (S1) travels a distance of 3.5 wavelengths. The wave from Source 2 (S2) travels a distance of 4 wavelengths to reach the same point. The difference in distance traveled by the two waves is one-half a wavelength; that is, the path difference is 0.5 λ.