When there is a rainbow, why is the sky inside the arc a lighter colour/brighter than the sky outside of it?
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You see a rainbow when the sun is behind you and the rain is ahead of you.
Inside the arc, *lots* of the sunlight, in *all* colors, is bouncing off the raindrops and coming back in your direction. *This makes the sky bright.*
Outside the arc, *much less* of the sunlight is bouncing off the raindrops back to you; mostly it’s getting scattered randomly or passing through. *This makes the sky darker.*
In the arc itself, the amount of sunlight that comes back depends on the exact angle and the frequency (color) of the light. *This makes the rainbow.*
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Keep looking at the rainbow. Notice that it’s part of a circle. (If you’re in an airplane, you might see the whole circle; but if you’re on the ground, just part of it.)
The sun is still behind you; the rain ahead of you. Imagine a straight line from the sun, to your head, to the raindrops. That line is going to pass right through the center of the rainbow circle!
Yes, that means you see a different rainbow from everyone else — just as when you look in a mirror, you see a different reflection from everyone else. The rainbow isn’t really “out there” in the sky, just as a mirage on a hot highway isn’t really a puddle “out there” on the road, and a mirror image isn’t really “in” a space behind the mirror’s surface. That’s what people mean when they say it’s just a trick of the light. The rainbow is really an image that’s being projected into your eye, not something that’s out there taking up space in the world.
This also means that when you see a rainbow touch the ground, there’s really *not* a spot on the ground that is “at the end of the rainbow”. If you move closer to where the rainbow seems to touch ground, the rainbow will seem to move away from you.
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