# The optics of vortex (in water): why there is a bright ring?

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.

• @Pablowako The Answer of yours helped me to realize that there is actually rainbow's on the picture of physics girl; there is bit blue near the orange, and there is bit orange near the blue. I think you can't do a rainbow with magnifying glass. Commented Dec 14, 2015 at 19:06
• Yes, depth does matter. But note that in the pool vortices, the surface curvature is actually much less pronounced than in the drawing, so the refraction of the light rays is also much lower. But that combined with a depth of about 3 meters will create such a shadow. Commented Dec 14, 2015 at 19:36
• @rodrigo I get the same shadow in all depths; 0.01 m to 3 m Commented Dec 14, 2015 at 19:42
• Jokela : check out the Falaco soliton. That's what this thing is called. Commented Dec 14, 2015 at 20:02
• @JohnDuffield Thank. I actually knew that they are Falaco Solitons, but then, I actually hadn't seek under that name about the optics. There is stuff to be found; like surface is a catenoid etc, even pictures and written texta about the optics. cartan.pair.com/carfre10.htm And this could explain the shade. But it can't explain the bright ring to be attached on the shade, or can it? Commented Dec 15, 2015 at 5:42

## 2 Answers

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.

The bright ring IS the black lines you drew: the light that is missing inside the circle is refracted towards the outer edge. When you focus light through a magnifying glass, there is a big round shadow and in the center a bright spot. This is because all the light that was supposed to be in the shadow is concentrated towards the spot in the middle.

In the experiment, the light that should be inside the circle is spread out evenly on the rim of the circle.

By the way, the black lines are an inaccurate representation of what is happening: the circle is a shadow. It is at the very bottom of the pool. The white lines are the light hitting the bottom of the pool. The light cannot travel any further.

And regarding the colour shift (red and blue), it happens because different light frequencies travel at different speeds in a medium and thus refract less or more, much like in a prism. Red light tends to refract the least.

• This explains the colour bit more thoroughly. en.wikipedia.org/wiki/Prism Commented Dec 10, 2015 at 18:40
• Thank you very much for your answer. It helps me to think this further (The colour). At this video; youtu.be/D2ym8wt5NWo?t=57 you see how the focal length has an influence to the brightness of the bright spot. And as you see, this brightness can be spread far wider than the original shade diameter. So the difference with this vortex shade is, that the ring is always exactly around the shade on all focal lengths. You answer should also suit this case; physics.stackexchange.com/questions/222916/… Commented Dec 10, 2015 at 20:23
• If you ask that in a different question ill answer it with pics Commented Dec 15, 2015 at 16:53