Why is a Rainbow?
The beautiful colors of a rainbow are perceptions, like all colors. However, the process that creates the light we see as a rainbow is actually based on some fairly simple math.
That math is what we call geometry. The light from the sun is reflected by the surface of raindrops in the sky (just like it reflects off a window or surface of a lake) and its direction also changes when it enters and leaves the raindrop. The combination of all those changes in direction means that most of the light coming out of the raindrops is at an angle of about 41-42 degrees away from the shadow of our head (or in the direction half way between your nose and your ear when you look directly away from the sun). That alone would make a bright white circle in the sky. The reason the rainbow has its colors, is that the different wavelengths of light, producing different color perceptions, are reflected back to our eyes at slightly different angles. This makes the white light of the sun smear across the sky with different colors produce as the light is smeared and then we have a beautiful rainbow to see.
You might also see that the sky inside the curve of a rainbow is always brighter than the sky outside. This is also because of the way light reflects through the raindrops. You can even see that in the rainbow I made with my garden hose.
You can find a more detailed explanation of rainbows at this website from Dartmouth. And remember that rainbows are rarely seen since you need to have rain drops in the sky off in the distance in front of you and the sun low in the sky behind you; all at the same time!
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Updated: Apr. 19, 2011