The other day, I walked out my front door and there was a huge double rainbow looming over the hill across the road. I had to stop and stare and, although I wasn't quite as moved as the fellow in this now-famous Double Rainbow video
, it got me wondering just what causes these beautiful things to form. It turns out that there's a little more to it than the simple mix of sun and rain that I learned about as a child.
First, let's take a look at light itself. While light is fundamental to almost every aspect of human life — from how we see, to when we sleep, to the food we eat — it has always been somewhat mysterious. We now believe that light has a dualistic nature, acting as both a wave (a disturbance, like a ripple, moving through space) and a particle (a discrete packet traveling through space). When light is acting as a wave, it moves along with certain characteristics such as wavelength (the distance between each peak in the ripple) and frequency (how many peaks pass by in a given period of time). Those characteristics are what give light its color. Sunlight typically encompasses all the wavelengths mixed together, some of which we can see and some we can't.
If there is moisture in the air, sunlight may hit water droplets as it enters the atmosphere. When this happens, the light enters the water drop and becomes refracted, meaning that its course is slightly altered. The degree to which the course is altered depends on the light's wavelength. Since sunshine contains all the wavelengths (and, therefore, all colors), when it hits the water drop, each wavelength is refracted at a slightly different angle causing the colors to fan out slightly. The waves continue through the drop until they hit the far side where they are reflected. While refraction diverts the light, reflection bounces it back, sending it in the same general direction from which it came. The waves pass back through the front side of the droplet where they are refracted again, spreading the colors further before they pass back out into the sky.
How rainbows get their distinctive shape
The arc of the rainbow has to do with the water drop itself. First, because the drop is spherical, it creates a round reflection. The entire rainbow is actually a circle, half of which is hidden by the earth. Second, the physical properties of water influence the refraction of the light. With freshwater, like rain, the most intense beam of light exits the drop about 42 degrees from where it entered, giving the bow its distinct arc. If you happened to see a rainbow made with seawater (in the spray from a boat, for example), you'd find that the arc was narrower because saltwater refracts the light more strongly. This angle also helps determine how and where we see a rainbow. The lower the sun is in the sky, the more of the rainbow we see.
The elusive double rainbow
Sometimes, not all of the light wave's energy is reflected. When this happens, the remaining light can reflect a second time, exiting the droplet at a slightly different angle. Now we see the bright primary rainbow and above it a fainter secondary rainbow whose colors are reversed.
If you look closely, you'll also see that the sky between the two rainbows seems unusually dark — a phenomenon called Alexander's band, named for Alexander of Aphrodisias, a Greek school who first described it almost 2,000 years ago. Again, this dark band occurs because of the water drop's properties. The light is concentrated along two specific angles, the primary rainbow at about 42 degrees, and the secondary rainbow at about 50 degrees. Very little light is left between the two and so the sky here appears abnormally dark.
To catch a rainbow yourself, wait for a slightly rainy day when the sun is less than 42 degrees from the horizon (the late afternoon is a great time to watch). Put your back to the sun and watch for that tell-tale glimpse of glorious color. Happy viewing!
Photos: WireLizard/Flickr, Bods/Flickr, Oyvind Solstad/Flickr