Rainbows form when droplets of water are hit by the light of the sun at just the right angle. Although it looks like an actual object, a rainbow is really just a beam of special, colorful light in the sky!
If you were to see a rainbow from up in the sky (like a bird or as a passenger in a plane), you'd be able to see that a whole rainbow is actually a full circle! The reason you see only a half-circle on the ground is that the horizon cuts off your view, making it look like the rainbow is something that starts and ends at some point off in the distance, when in fact, it's something floating up in the sky!
The "horizon" refers to the place where it looks like the land meets up with the sky. In fact, the land never meets up with the sky, since the sky wraps all around the Earth! And since a rainbow is up in the sky, and the sky wraps all around the Earth, you can think of a rainbow like a big ribbon on top of the Earth, wrapping it up like a present!
by Mya Kagan (Whyzz writer)
Sometimes, you won't see an entire half-circle rainbow and you'll just see a small section of one. This happens when the light hitting the droplets in the air only hits a smaller section of water droplets.