The Mystery of Double Rainbows

Double rainbows: If the hope of one pot of gold isn't enough, sometimes Mother Nature gives us the opportunity to pursue two. This double-dose of atmospheric optimism is a result of optical effects occurring in tandem.

All rainbows require the presence of the sun and rain in order to form. In order to see a rainbow, the sun must be to the viewer's back, and rain must be falling ahead of the viewer. It doesn't necessarily have to be raining on or near the viewer, but rain must be present ahead of the viewer towards the horizon. And during a particularly lucky scenario, two rainbows will form at the same time.

As sunshine breaks through the clouds and beams towards the raindrops, some of the light encounters the raindrops and bends - called refraction. When the light refracts, the process causes the sunlight to separate into different wavelengths. These different wavelengths correspond to different colors: red and orange correspond to longer wavelengths, while blue and purple correspond to shorter wavelengths. 

The refracted light waves then bounce - or reflect - off of the circular edge of the raindrop, and then they refract again as they exit the raindrop and travel through the air.

Because raindrops are relatively round when the sunlight refracts through them, the visual result is a spherical arc that soars all across the sky. Viewers who are lucky enough to see a whole rainbow will observe a colorful arc spanning the entire Earth, from end to end.

Rainbows remain a relatively rare event as they will only occur when the refracted sunlight strikes the raindrop's edge at the exact angle of 48 degrees. If the angle is less than 48 degrees, then the light will simply pass through the raindrop. Any greater than 48º, then the light reflects straight back out of the raindrop, and no refraction will occur. Without refraction, a rainbow will not form.

When two rainbows form at the same time the first and brighter rainbow is called the primary rainbow. This rainbow is created by the process described above, and only requires the light to reflect off of the raindrop once before refracting out of the raindrop. 

The second and fainter rainbow is called the secondary rainbow. It occurs when refracted light does not escape the raindrop after being reflected the first time. Instead, the refracted light reflects off the raindrop's surface a second time as well, producing a secondary rainbow with its colors reversed compared to the primary rainbow. Unfortunately, fewer light rays are available to undergo the additional refraction process, so the resulting secondary rainbow appears less vivid.