This is a top view of a ripple tank. Initially there is just one point wave source. Click on the run button to have the program calculate the wave pattern. The initial display will be slow, but then the animation will run smoothly.

Note how the waves from the point source radiate outward evenly, resulting in a circular pattern. The wavelength is two units, which you can confirm using the mouse. Both peaks and troughs in the waveform show up as light colors, so you must measure between alternate white bands.

The control for the number of sources allows you to add additional identical sources, which will emit the same wave pattern from a different location. The slider labeled "Length of array" sets the distance between the first and last source, and is only meaningful when there is more than one source. For the interference of waves, the separation between the sources is of critical importance. For N sources, this separation is found by dividing the array length by N-1.

Set the number of sources to two. Interference between the two waves will create a new pattern. If a crest from one source overlaps with a trough from another, the resulting destructive interference will create a region of no displacement. You can see the interference pattern using the following steps.

The two sources are one unit, or half a wavelength, apart. How will the waves combine from this combination? Begin by predicting the wave pattern along the x axis. The distance from any point along the x axis to each source is always different by one half wavelength, which means that the crest from one source will coincide with the trough of the other. Will there be any wave amplitude anywhere along the x axis? Make a prediction, and check it out using the simulation.

Now consider the perpendicular bisector of the line between the two sources. Along the line the distance to each source is the same. Will a crest from one source arrive at the same time as a crest from the other? Look at the wave pattern to check your answer.

What would happen to the pattern if the two sources were exactly out of phase? Set this up by adjusting the phase difference to p. Make a prediction of the wave intensity along the x axis, and then test your prediction with the simulation.

Try other arrangements of sources, and see if you can understand the resulting patterns.