Shadows of Internal Gravity Waves

In this experiment, the tank was gradually filled with very salty water on the bottom and progressively fresher water on the top. When the fluid has a density that decreases uniformly with height like this tank, even small disturbances cause waves to move through the fluid.

When the motor moves the suspended cylinder up and down in the salt water it is enough to form internal waves that we observe as shadows on the wall (due to the changes in density). The faster the cylinder oscillates the steeper the X-shape. The waves reflect off the bottom of the container and the surface of the water resulting in a beautiful diamond pattern.

Check out Dr. Rhines' website for a more thorough and mathematical description:
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[dimk]

Thank you for demonstrating St. Andrew's Cross!

[michaelschwartz]

Shadow puppet waves, very interesting!