Click the link for locally archived simuations due to Falstad.

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Falstad's java Applet simulates ripple patterns in a shallow tank. It demonstrates interference, diffraction (single slit, double slit, etc.), refraction, resonance, and the Doppler effect.

Go through the items in the Setup menu in the upper right to get started with the Applet. The predefined setups are starting points; you can modify the sources and walls as you desire. You can draw on the screen with the mouse.

For the original try ... http://www.falstad.com/