Theory: Two nearby sources sending out waves at the same frequency will produce an interference pattern, the maxima of the pattern coming at angles given by d sin(theta) = n lambda.
Description: The tank is set up over the projector on a movable cart. The relative phase of the sources can be adjusted to change the pattern; the oscillation frequency is controlled by the voltage and the separation is set by hand. Tip 1 - the darker the room, the easier it is to see the pattern. Tip 2 - for best results, the overhead projector should be out of focus. It's best to have the lens closer to the tank then it would be when in focus.
Setup time: 20 minutes
References: (available in the demo room)