Recently, there was a discussion of why rainbows occur and also why the angle to the rainbow is always the same. So I've put together these two pages on rainbows.
Light is a wave.
If you drop a stone in a pond you'll see circular waves moving outward at an easily measured speed, let's call that v. All waves are characterized by these quantities. Generally we only need to specify two since the third is determined from this formula. This turns out to be about 300,000,000 meters per second. Well, that's what determines what we perceive as the color of light.