Reply #11: Alexander's dark band

[tekisui]

The light that causes the rainbow emerges out of water droplets at every possible angle up to the radius of the rainbow. So, the light that interacts with the droplets and which reaches your eyes, forms a disk of light, with the colored rainbow at its perimeter. Outside the rainbow, there is no light exiting raindrops in the way that produces the main rainbow. Hence this area appears darker than the area within the bow. The dark area outside the rainbow is called Alexander's dark band.

http://www.weatherscapes.com/techniques.php?cat=optics&page=rainbowfaq

Alexander's Dark Band:
http://en.wikipedia.org/wiki/Alexander%27s_band

I had never looked into it before. Thanks for the prompt.