Light Years

Seeing double: Researchers find rainbow connection

YouTube has taught us that a double rainbow may be worthy of an ecstatic rant. But there's a natural multicolored phenomenon even more spectacular: a twinned rainbow.

A twinned rainbow happens when the primary bow splits in two.  Scientists have found out more about how this rare event happens by using sophisticated computer science techniques.

Iman Sadeghi, a software engineer in Google's graphics division, did research on the physics of rainbows while pursuing his doctorate at the University of California, San Diego. He and his adviser, Henrik Wann Jensen, who worked on the James Cameron film “Avatar," collaborated with scientists from Spain, England and Switzerland. The team plans to publish the results in the journal ACM Transactions on Graphics.

Sadeghi and colleagues set out to reproduce different rainbows. They created computer simulations to recreate the geometry of a real raindrop and capture the behavior of light as it leaves water drops.

In a primary rainbow, light gets reflected from water droplets and then comes out. In a secondary rainbow, light gets reflected twice in the droplet before emerging.

But what about twinned rainbows? By looking at simulations, Sadeghi and colleagues realized that spherical water drops cannot possibly produce that effect.

Instead, you'd need a rain shower with both small spherical water drops and larger, flatter droplets that they call "burgeroids" because they look like hamburgers. The burgeroids shift light in one direction, while the small droplets produce a separate bow.

It takes a uniform distribution of large drops of water of the same size to produce a twinned rainbow, Sadeghi said. Since that's usually not the case, twin rainbows are rare.

This research is "the first scientific, accurate simulation of twinned rainbow in history," Sadeghi said. "We ended up pushing the edge of rainbow science."

These findings might be useful for a film or video game maker concerned with scientific accuracy in the portrayal of rainbows, although typically those media don't pay attention to such details, Sadeghi said.

Sadeghi's method is a lot slower than just copying an existing photo of a rainbow - each water drop you want to simulate takes six hours, he said.

"The application is very narrow for specific people who care about the appearance of rainbows," he said. "But if a movie wants a twin rainbow, they should definitely come to us."

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