Alexander's Band Is The Dark Strip Of Sky Inside A Double Rainbow
It's hard to suppress the knee-jerk, childlike giddiness of seeing a rainbow. Though you've probably seen plenty of these colorful arcs in your life, getting a clear view of a double rainbow is less common, and decidedly much more wonderful. (Just ask 2010's Double Rainbow Guy; remember him?) But don't be too distracted by the colors to notice the dark rainbow sandwich also stretching across the sky.
"Double Rainbow All The Way!"
They always seem pretty magical, but rainbows really aren't anything too wild or rare. The ingredients for a primary rainbow (semi-fancy science term for regular ol' rainbow) are raindrops and sunlight. Put simply, rainbows happen when raindrops refract (bend) and reflect light. Think of it like this: Light will twist and its colors will separate when it enters a raindrop, the light will bounce off the outer boundary of the drop, then it will all twist again as it leaves the droplet.
Double rainbows (or primary rainbows with visible secondary rainbows, in boring science-speak) are just the result of extra bouncy light, basically. When light rays escape a raindrop after two reflections, two rainbows are visible. The secondary rainbow is almost double the width of the primary bow, and its colors are reversed — though much fainter — so that the reds of the two rainbows always face one another. Because the light gets an extra bounce in the raindrop, they flip again, which causes the color swap.
Fun fact: You could hypothetically have a triple or more rainbow, but those extra arcs would be way too close to the sun to be visible. Still, it's kind of magical knowing quadruple rainbows are out there, isn't it?
Read Between The Lines
The colors of a rainbow are clearly the headlining act, but there's a cool story going on around the edges, too. During a double rainbow, the stretch of sky between the two colorful arcs will appear darker than the rest of the sky. This is called Alexander's band, named after Alexander of Aphrodisias, who first noted the phenomenon in 200 A.D. (He had no idea what was going on with it, but still, credit where credit is due.) Because light bounces off the back of a raindrop and emerges on the same side it entered, the area of sky below a rainbow is the brightest. The same thing happens in the secondary rainbow as a mirror image, so the light is brightest above the faint rainbow. This leaves you with a rainbow sandwich of dark sky.
For the harder science folk, here's a more technical explanation by Earth and space scientist Jon Plotkin of Decoded Science: "The angle made by the line from an observer to the center of a primary rainbow and a line from the observer to the rainbow's arc is 42°. The comparable angle of the secondary rainbow is 50°. Between these angles, no light can be scattered to the observer by primary or secondary reflections. Since light can be scattered from this region by other reflections and refractions, the band is not completely dark."
Admittedly, this is less catchy than Double Rainbow Guy's emotional breakdown, but it's worth noting.