I occasionally see a double rainbow in the sky. What causes two rainbows to appear sometimes? asks reader Jean, via emailIf you've ever been lucky enough to see a double rainbow, you may have noticed that the two rainbows, arcing side by side across the sky, aren't exactly twins. One bow is brighter, and its top band of color is red. The second rainbow is dimmer, and its colors reversed -- the red band is at the bottom.
A rainbow is a kind of trick of light, and the conditions must be just right for us to see one. Sunlight must be streaming from behind us, so the sun must be low on the horizon. And in front of us, of course, there must be a curtain of falling raindrops.
What happens between light and drop? When a ray of sunlight pierces a raindrop, it begins to bend. That's because the light is passing into water, which is much denser (its molecules more closely packed) than thin air. White light is really a blend of hidden colors -- red, orange, yellow, green, blue, indigo, violet (aka Roy G. Biv). And as a light beam enters a drop, each color, or wavelength, bends (refracts) a little more or less. The result? The white sunbeam splits apart into its true colors, each going off on its own direction inside the drop.
The colored rays collide with the mirror-like raindrop wall, bend again, and shoot back out of the drop. Making their escape, the colored rays bend again as they move from denser water into open air.
While all raindrops in the sky refract sunlight, we're only able to see light coming from drops at a certain viewing angle. From this rainbow band of drops, our eyes receive different colors from drops at different positions in the sky.
Since red and orange wavelengths are bent the most, we see these colors courtesy of higher drops. Blue and violet are less sharply bent, so we get these colors from lower drops. Yellow and green are provided by the drops in the middle of the band. The result is an arc of color, stretching across the sky.
This bright, "primary" rainbow is usually the only one we see. So what causes a secondary rainbow to materialize? The second bow appears when light rays enter near the bottom of each drop, and then reflect from the inside wall of a raindrop twice (instead of once) before exiting. When these twice-reflected rays emerge from many different raindrops, we see a second rainbow, paler and wider, above the first.
The second bow's colors are reversed. Why? Doubly-bent red light from the highest drops ends up traveling over our heads. So we see violet light, bent to our eyes from lower raindrops, as the first band of light, followed by indigo, blue, green, yellow, orange and red.