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.

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).