Smithsonian.com

From the researchers’ paper, the twinned rainbow appears on the far right. Photo: Wojciech Jarosz

Who hasn’t been swept away by the stunning beauty of a double rainbow? Don’t get too excited, though, apparently there’s an even more rare and “exotic” phenomenon called the twinned rainbow, in which two rainbows appear to split from a single stem. Though much of the physics behind rainbows is still cloaked in mystery, researchers have at last unlocked some of the secrets behind the peculiar optics of the twinned rainbow.

To unravel the odd occurrence of the twinned rainbow, scientists studied virtual rainbows in simulations that modeled differently shaped water drops, taking into consideration the dual particle and wave-nature of light. The key behind the twinned rainbow, the researchers claim, is the combination of different sized water droplets.

Previous simulations assumed that raindrops maintained a spherical shape as they fell to Earth, but the researchers in this study realized that, as rain drops plummet, air resistance flattens them into the shape of a hamburger. Appropriately, these drop are dubbed “burgeroids.”

When two showers occur simultaneously, their models show, different sized drops can create “slightly deformed” rainbows, like the double-headed twinned rainbow.

The researchers were not originally searching for this pot-of-gold finding at the end of the twinned rainbow. Rather, their work was supported by the Walt Disney Company, which is on a quest to create accurate-looking rainbows for animated movies and video games.

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