MetaFilter: “Ripple Tank Simulation is a delightful, mesmeric java applet simulation of a ripple tank. It demonstrates two dimensional wave phenomena such as interference, diffraction, refraction, resonance, phased arrays, and the Doppler effect (do try the 3D view). From Paul Falstad’s fantastic collection of Math, Physics and Engineering Applets.”

Excellent, I’m adding the ripple tank – and the other applets – to my collection of useful sites for physics lessons.