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Scientists
have yet to fully unravel the mysteries of rainbows, but a group of
researchers from Disney Research, Zürich, University of California, San
Diego, Universidad de Zaragoza, and Horley, UK, have used simulations of
these natural wonders to unlock the secret to a rare optical phenomenon
known as the twinned rainbow.
Unlike
the more common double-rainbow, which consists of two separate and
concentric rainbow arcs, the elusive twinned rainbow appears as two
rainbows arcs that split from a single base rainbow. Sometimes it is
even observed in combination with a double rainbow.
It
is well-known that rainbows are caused by the interaction of sunlight
with small water drops in the atmosphere; however, even though the study
of rainbows can be traced back more than 2,000 years to the days of
Aristotle, their complete and often complex behavior has not been fully
understood until now.
“Everyone
has seen rainbows, even double-rainbows, and they continue to fascinate
the scientific community,” said Dr. Wojciech Jarosz, co-author of the
paper and Research Scientist at Disney Research, Zürich. “Sometimes,
when the conditions are just right, we can observe extremely exotic
rainbows, such as a twinned rainbow. Until now, no one has really known
why such rainbows occur.”
Jarosz
and the international team of researchers studied virtual rainbows in
simulation, considering the physical shape of water drops, and their
complex interactions with both the particle and wave-nature of light.
The key to the twinned rainbow mystery, Jarosz said, is the combination
of different sizes of water drops falling from the sky.
“Previous
simulations have assumed that raindrops are spherical. While this can
easily explain the rainbow and even the double rainbow, it cannot
explain the twinned rainbow,” he said. Real raindrops flatten as they
fall, due to air resistance, and this flattening is more prominent in
larger water drops. Such large drops end up resembling the shape of
hamburgers, and are therefore called “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 team developed software able to reproduce these conditions in
simulation and the results matched, for the first time, twinned rainbows
seen in photographs. The team also simulated a vast array of other
rainbows matching photographs.
The
team’s discovery was unintentional. “Initially the goal was to better
depict rainbows for animated movies and video games and we thought
rainbows were pretty well understood,” said Jarosz. “Along the way we
discovered that science and current simulation methods simply could not
explain some types of rainbows. This mystery really fueled our
investigations.” The researchers now see potentially wider application
of their method beyond computer graphics, speculating that, some day,
accurate rendering models of atmospheric phenomena, like the one they
developed, could have impact in areas such as meteorology for deducing
the size of water drops from videos or photographs.
The
research findings by will be presented Aug. 8 in the “Physics and
Mathematics for Light” session at SIGGRAPH 2012, the International
Conference on Computer Graphics and Interactive Techniques at the Los
Angeles Convention Center.
Research paper: Physically-based Simulation of Rainbows
Source: Disney Research
