An international team of scientists working with Disney have figured out how twin rainbows form while trying to enhance rainbow images in computer graphics.
© Iman Sadeghi, Adolfo Muñoz, Philip Laven, Wojciech Jarosz, Francisco Seron, Diego Gutierrez, Henrik Wann Jensen
Rendering results for different types of rainbows: (a) Rainbow derived from Lorenz-Mie theory; (b) Single primary rainbow with considering the angular view of the sun; (c) Double rainbow with a flipped secondary rainbow; (d) Multiple supernumerary rainbows caused by small water drops with uniform sizes; (e) Twinned rainbow resulted from mixture of non-spherical water drops and spherical ones.
Twin rainbows are rarer than double-rainbows - separate concentric arcs - instead appearing as two arcs splitting from a single origin.

"Initially the goal was to better depict rainbows for animated movies and video games and we thought rainbows were pretty well understood," said study co-author Wojciech Jarosz at Disney Research, Zürich, in a press release.

"Along the way we discovered that science and current simulation methods simply could not explain some types of rainbows," he added. "This mystery really fueled our investigations."

The researchers simulated rainbows, while looking at the effect of drop shape, and interactions with light both as a particle and a wave. They found that this rare optical phenomenon arises due to different-sized raindrops, and were also able to create others like supernumerary bows.

"Previous simulations have assumed that raindrops are spherical," Jarosz explained. "While this can easily explain the rainbow and even the double rainbow, it cannot explain the twinned rainbow."

As rain falls, air resistance causes the droplets to flatten, especially if they are large, producing hamburger-shaped drops known as "burgeroids."

"Sometimes two rain showers combine," Jarosz said. "When the two are composed of different-sized raindrops, each set of raindrops produces slightly deformed rainbows, which combine to form the elusive twinned rainbow."

The findings will be presented in Los Angeles at SIGGRAPH 2012, the International Conference on Computer Graphics and Interactive Techniques, on Aug. 8.