Ripple Tank Lite

A simulation of a ripple tank, or wave tank. Make waves with your finger, or view examples including such wave phenomena as interference, diffraction (single slit, double slit, etc.), refraction, resonance, and the Doppler effect.

There is also a full version of this app, called "Ripple Tank", which allows you to create your own experiments, save and load, and change colors.  It is also optimized for iPad.