Rainbows are actually circular but because we see them from the ground we typically only see half of them. If you view a rainbow from an airplane you can see the whole circle.
But why?
Light spreads out a bit, and it does so consistently and evenly.
Consider a flashlight shining on a wall ten feet away: the small circle of light spreads out into a large circle of light.
That’s sort of what we’re seeing in a rainbow, that expanded circle of light as it passes through airborne water.
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