I saw a circular rainbow on my recent early morning flight. In the center was violet and then the rainbow colors spread out in a circular fashion following the light spectrum (violet, blue, green, yellow, and then red). Researching the phenomenom, I discovered that it's technically not a rainbow, but instead a glory.
There seems to be some debate over the exact nature of what causes the visible light to separate into the colors, but it seems clear that the sunlight interacts with water droplets in clouds and travels back.
It was a particularly striking image when the plane began its descent. The shadow of the plane fell in the circle of color, and as we moved, the distance to the clouds changed. While the glory size didn't change, the plane's shadow grew when the clouds were closer. In fact, glory appears to be smaller, but that's due to optical processing. When something is closer, it should be bigger, but the actual glory size doesn't change, so it's perceived to be smaller on the closer clouds. This reference point effect is probably similar to why the moon appears larger on the horizon.
Putting aside causes and effects, I value the moment; seeing the image was new and completely unexpected.
You can read more and see images of Glories on Wikipedia.
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