This picture is from this YouTube video from "Physics Girl", which shows how you can generate vortices in a swimming pool by using a plate:
And there she explains the creation of the shaded circle due to one of the vortices. But as you see, there is also a bright ring, which is there no matter the depth of the water. The red arrow points to what looks like a wing.
The white lines in the image above are the paths of light ray, as they are deflected by the vortex (see image below as well). But shouldn't the light paths continue past that? Or also through the middle of the vortex? The black lines in the picture above (added by me) and the white lines (below, on right) should show what I mean.
The water depth should also have an influence on the brightness of the ring, right? That is to say, with very deep waters, there shouldn't be any bright ring visible. So does the depth really matter here?