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?
Here's a foto from experiment showing that these different shade types are also possible.
Best Answer
I was hoping to get some good answer, but as It's not present, and the one given, has not got too many upvotes, I dare to Say my own answer, though it might be considered "not mainstream".
The optics is created similarily like in a rainbow;
At rainbow there also a shade, but it's just outside;
At the vortex there is a vortex ring / Toroidal vortice at the top of the Vortex, and this causes the reflection as seen. Between the forced vortex and Free vortex the fluid is "cut" and there is a surface inside the fluid which reflects the light. This Vortex ring is a forced vortex in top of the forced vortex filament.
This causes the light to bend "rainbow like"; Meaning here that the bright light is exactly in the border of the shade, and not like in a lense, where the shade and light are not attached at all.
The existence of this toroidal vortice, and it's reflecting surfaces can been proven by putting the light below the water surface (Left picture below);
As seen the air hose, shade goes through the shade&light-ring. These shades can be seen in these videos.
As the light needs a surface to been bent, it means that there must be surfaces inside the fluid. And this is the answer for the question; "what is Turbulence?"; Would a solution to the Navier-Stokes Millennium Problem have any practical consequences? It also fills the cab with Newton's basic laws, as it' gives a physical explanation for the creation of this rotation. Which according to Newton's first Law shouldn't be possible.