Why is violet on the bottom of the rainbow?

There are a couple of complicating factors in answering this question, so bear with me. First: a rainbow is actually a complete circle, or would be if the ground didn't get in the way. So instead of saying "bottom" and "top" let's talk about the inner arc and the outer arc. Second: the physics of refraction works out so that there are sometimes two rainbows; the outer one is considerably dimmer than the inner so it's often not noticeable. See the pictures on the Wikipedia rainbow page in Related Links for several examples. The secondary bow has the colors in reverse order. I'm only going to talk about the primary bow. Those caveats out of the way, the color on the inside of the primary bow ("bottom" at the highest point of the arc) is violet.