How often have you seen two rainbows at the same time? Correspondent Bob Henson found himself wanting to learn more about the unusual phenomenon of double rainbows.
During my first summer in Colorado, time and again I found myself captivated by not just one brilliant rainbow but another right above it. I'd seen occasional snippets of a double rainbow when I lived in Oklahoma, but never such glorious, brilliant arcs as these. I noticed the top bow was dimmer and the sequence of colors was reversed: red on the bottom, violet on top. It was only recently that I learned why double rainbows happen.
When a beam of sunlight hits a raindrop, it gets refracted on the way in and on the way back out. The angles are so dependable that you can use the location of the sun to calculate exactly how high a rainbow will be. It turns out that a few of the light beams bounce off the back of the raindrop not once, but twice. It's these secondary rays of light that end up leaving at a different set of angles and forming a second rainbow about 9 degrees above the first one. Since only part of the light does this double journey, the second rainbow is always dimmer than the first. This makes it easier to see the second rainbow in places where the air is especially clear, like it often is at Colorado's high altitude.
Is there a third rainbow? In fact, there is, but it's weaker still, and it forms on the opposite side of the storm, around the sun itself, so sunlight blots it out. That's fine with me. If I can get two gorgeous rainbows out of one storm, I see no reason to push my luck.