The ripple tank starts with two sources, in phase. It will calculate 7 frames of the animation and then run it. If you change to one of the other views, it will need to recalculate before it will run. The ripple tank has a wavelength of 2 units. Pick one of the "darks" and calculate the path difference of the signals from each source - see how many wavelengths fit into that path difference ... then try for one of the "brights".
Physlet problem originally authored by Wolfgang Christian. Modified by Dr. Scott Schneider