its called “Alexander’s band” it has to do with the way water is bending the light. if you think about how a rainbow works…….the light from the sun is being bent by water in the air; this bending “messes up” our viewing angles
[https://en.wikipedia.org/wiki/Alexander%27s_band](https://en.wikipedia.org/wiki/Alexander%27s_band)
A rainbow lines up with you and the sun. If you were in the air above a waterfall, for example, you could see a complete ring.
The water droplets inside the rainbow act like a mirror and reflect light back at you. As you move outward through the rainbow, the light is hitting the drops at an angle, relative to you, so it acts like a prism and refraction the light.
The light that hits the water outside the rainbow is reflected away from you.
It’s due to the same mechanism that creates the rainbow itself!
Light reflecting within and refracted by water drops in the air gets sent “back” toward the viewer, resulting in a light area of sky. On the edge of that, the light is sorted by frequency, and shows up as bands of color (a rainbow!). Past that, the light is directed *away* from the viewer, resulting in an area that looks darker because there’s less light coming at you from it.
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