Internal waves are gravity waves that oscillate due to the buoyancy force. The simplest example are the waves generated on the interface between two fluids of different densities. These waves typically have much lower frequencies and higher amplitudes than surface waves. This is because the density difference between two fluids is typically much less than the density difference between water and air. A good way to visualize internal waves is to observe the waves on an oil-water interface.

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Directions for making an oil and water wave toy.