Ripple Tank (2-D Waves) Applet
Displays wave interference from a wide range of source and barrier configurations. The user can select specific configurations or add sources and barriers themselves.
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Matt Johnson (Student)
A fantastic tool. I find it can be used over and over again while teaching waves to provide a quick visualization of exactly what a property mans.
Fairly processor heavy. Certainly you will to download and attempt to run on your in class machine before getting the kids hopes up in class.