Blink and you might miss them, but these two colorful smudges (center) form the second half of the first quadruple rainbow to be caught on camera, validating years of claims that such phenomena could exist, a new study says.
"[It's as if] the natives are telling you that there's this creature in the rain forest, but they only have stories," said Raymond Lee, a professor of meteorology at the U.S. Naval Academy, who recently published a study in the journal Applied Optics describing the conditions under which a tertiary rainbow should be visible.
Lee's research inspired some scientists to try to snap photos of the elusive vision, resulting in this picture, taken June 11 near Bremerhaven, Germany. "When you finally succeed in capturing it in photography, that's a thrill," Lee said.
And while "quadruple rainbow" might call to mind a stack of four arcs, only two rainbows can be caught in a single frame, because of the way light reflects and bends within raindrops.
As a ray bounces around inside a droplet, some light escapes and is split into its constituent colors, forming a rainbow. Each time the ray gets reflected, the light, and thus the rainbow, gets a bit dimmer.
When this happens three or four times, the final rays exit in the direction of the light source—the sun—and create extremely faint rainbows opposite the original two, which can be seen only while facing away from the sun.
"[There are] a couple conditions that are conducive to forming these things. Neither one of them is very enticing for photographers or casual viewers," Lee said. "You have to have ... an absolutely inky black cloud background ... and then either a uniform distribution of raindrop sizes, or it has to be absolutely pouring."
These uncomfortable conditions may explain why triple and quadruple rainbows have been so hard to find for so long, despite having been known in theory for more than a century.
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