In the salad dressing bottle, the available space is a little too small to tell how the waves are moving, so let's pour oil and water into a much larger water tank, and make waves on the boundary. Of course, when you cause waves at one corner of the tank, the waves spread in a concentric fashion. This is the same as normal waves on a water surface. This photo is a view from above those waves. Even if waves do occur at the boundary, however, it is difficult to see them because both the oil and water are transparent. So, we used a trick to cause a color change according to the height of the boundary surface.