Why some rainbows are all red

Sun’s spot in the sky influences how light bends to form colorful arcs


ON THE SPECTRUM  Rainbows containing only a limited fraction of the light spectrum form as the sun drops lower in the sky, new research finds.

Graham Hickey/Flickr (CC BY-SA 2.0)

SAN FRANCISCO — Some rainbows don’t contain all the colors of the rainbow. The height of the sun above the horizon can yield arcs that contain only a fraction of the traditional ROYGBIV, researchers reported December 17 at the American Geophysical Union’s fall meeting.

Rainbows appear when sunlight bends as it passes through airborne water droplets. Different parts of the light spectrum bend by different amounts, breaking the light into its individual color components and creating a colorful arc.

On rare occasion, a rainbow will contain just part of the color spectrum. Previous work suggested that these less colorful or even monochromatic rainbows form primarily due to the size of the light-bending water droplets. Atmospheric scientist Jean Ricard of the National Centre for Meteorological Research in Toulouse, France, and colleagues cataloged hundreds of photographs of rainbows. They discovered that the sun’s position plays a much larger role than previously thought. During sunset, the sun’s low point means light has to travel a longer distance through the atmosphere. During the journey, the air scatters away the cooler colors. The redder remaining light thickens a rainbow’s red band, eventually squeezing out the violet, blue and orange band and leaving behind a red-only rainbow.

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