This simulation demonstrates interference of waves from two identical sources that are separated by a variable distance. The wave crests are shown in red and the troughs in blue, with black indicating a local wave amplitude of zero. Alternatively, you can switch to black-and-white, using the Grayscale button - in that case both the peaks and troughs are white. (Note that the entire image updates instantly when you move the slider, because it is drawn using a formula that assumes fixed separation, rather than from a simulation of the time-dependent response of the wave medium. Note also that the wave amplitude does not decrease with the distance from the source, as it would for physical waves.)
You can click-and-drag on the yellow point to change its position. When is there constructive interference at the yellow point, and when is there destructive interference at that point?
The graph shows the displacement at the yellow point because of the left source (the pink graph), the right source (the orange graph), and the net displacement (in green).
This version of the simulation first posted by Andrew Duffy, 8-11-2017.
Last uploaded 8-14-2017, to add the grayscale option.