Internal reflection of a vortex ring at an air-water interface

A vortex ring is a structure of fundamental importance in fluids, which can also be observed in other fields, including plasmas and electromagnetism. The interactions of a vortex ring with a solid boundary, a free surface, and other vortex rings have been widely studied. However, the intriguing phenomenon of reflection of a water vortex ring at an air-water interface has not drawn enough attention. We investigate this problem experimentally and observe vortex ring reflection that is similar to the total internal reflection of light. Based on our observations, we develop a simple model that explains the phenomenon.