Same applet, bigger size
|The interference pattern relies on the wavelength, the source-separation and whether the two sources are in phase or not.|
|Click any point on the interference pattern, the two waves coming from the sources to that point will be shown. The waveforms coloured in blue are equal in their paths of travel and that in red is the waveform along the path difference.|
In particular, when the two sources are in phase (phase difference = 0), we only need to look at the phase relationship between the black and the red dots (or the green and the red dots if the upper path is longer) . If the black and the red dots are in phase, a constructive interference occurs at P; but if they are in antiphase, the interference at P is destructive.
In terms of path difference,
constructive interference occus at P when path difference/wavelengty = 0, 1, 2, 3, .... and
destructive interference occurs at P when Path difference/wavelength = 0.5, 1.5 , 2.5,... ..
Don't forget that these conditions are only valid for two in-phase sources.