A rainbow is a multicolored arc made by light striking water droplets.
A rainbow is an optical illusion and does not actually exist as a standalone phenomenon although visible to us. The appearance of a rainbow depends on where you’re standing and where the sun is shining.
Rainbows are formed when light rays enter a water droplet and are are bent (or refracted) inside the droplet until the ray hits the back of the droplet which reflects it. As this reflected light leaves the droplet, it is refracted again and spread out across a wider area. It is this that forms what we see as a rainbow.
Full article originally published in National Geographic.