Cork balls, Edwardian magazines, and optical physics

Okay, let’s look at a really cute physics demonstration that I found in a really unusual place and which has unexpected connections to optical physics! The demonstration in question is set up as follows: attach a string to a 1 inch diameter cork ball, and swing the ball into a stream of falling water from a tap. The result? The ball feels almost magnetically attracted to the stream of water! A photo of my attempt at this is shown below. Note that the string is at an angle. If it were free to swing, one would expect the ball to fall out of the water till the string is vertical. In fact, one might intuitively expect that the ball would be forced out of the stream of water, but instead it is held within it! In case you think this is a trick of still photography, here is a gif of the phenomenon. As we will see, this trick has been around for quite a long time, and it is not only a great nonintuitive demonstration of some fundamental physics but can be used to illustrate a really important…