Computer simulations shed light on the physics of rainbows

Public release date: 6-Dec-2011
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Contact: Ioana Patringenaru
ipatrin@ucsd.edu
858-822-0899
University of California - San Diego

Computer scientists at UC San Diego, who set out to simulate all rainbows found in nature, wound up answering questions about the physics of rainbows as well. The scientists recreated a wide variety of rainbows primary rainbows, secondary rainbows, redbows that form at sunset and cloudbows that form on foggy days by using an improved method for simulating how light interacts with water drops of various shapes and sizes. Their new approach even yielded realistic simulations of difficult-to-replicate "twinned" rainbows that split their primary bow in two.

UC San Diego alumnus Iman Sadeghi, who did the work while a Ph.D. student at the Jacobs School of Engineering, his advisor, computer science professor Henrik Wann Jensen, and scientists from Spain, England and Switzerland, are set to publish their findings in ACM Transactions on Graphics in December of this year.

"This goes beyond computer graphics," Jensen said. "We now have an almost complete picture of how rainbows form."

Jensen is no stranger to advances in computer graphics. He earned an Academy Award in 2004 for research that brought life-like skin to animated characters. He has worked on a number of Hollywood blockbusters, including James Cameron's "Avatar."

Jensen, Sadeghi and colleagues originally set out to simulate rainbows to better understand how spherical water drops interact with light, resulting in the bright, multi-colored arcs that we are used to seeing when rain stops or in tropical, humid weather. They were hoping to improve techniques used in animated movies and video games.

"You usually don't get the opportunity to study such beautiful phenomena while working on your Ph.D. thesis," said Sadeghi, who is now a software engineer in the graphics division of Google in Santa Monica. "There is a lot more to rainbows than meets the eye."

As they started running various simulations, the scientists realized that the interaction of light with spherical drops could not explain some kinds of rainbows, such as twinned rainbows. Scientists turned to research showing that, as a water drop falls, air pressure flattens the bottom of it and shapes it like a burger. Jensen and his team called these slightly deformed water drops "burgeroids." "It's not a very mathematical term, but we like to use it," Jensen said. Simulations based on the so-called burgeroids, rather than on spherical drops of water, allowed the researchers to replicate a wide range of rainbows found in nature. "We are the first to present an accurate simulation of twinned rainbows," Sadeghi said.

The basic mechanism behind the formation of rainbows has been well understood for hundreds of years: A beam of light is both reflected and refracted within the water drop, and becomes strongly concentrated near the "rainbow angle" in the drop. The rainbow angle changes with the color of the light. As a result, sunlight separates into its spectral components, forming the colors we see in the sky. "The variation in the appearance of rainbows is due to the size and shape of rain drops" Sadeghi said.

It is surprising that the physics of rainbows are still not completely understood, Jensen said. In the past, eminent scientists, including Isaac Newton and French mathematician Rene Descartes, made calculations and conducted experiments to explain how rainbows form. But today, funding for rainbow research is scarce and so is work on the topic.

Jensen's quest to learn about the physics of rainbows led him to the Light and Color in Nature conference at St. Mary's College in St. Mary's City, Md. He served as keynote speaker and met Philip Laven, an internationally renowned expert on rainbows, who became one of the study's co-authors.

Until now, most simulations of rainbows had assumed that water drops are spherical, which isn't true for large rain drops, Laven said. In this paper, researchers have adopted a completely different approach and developed a more realistic model to recreate rainbows, he said.

"The simulations shown in this paper offer the prospect of a better understanding of real rainbows," Laven said. "I hope that the next step will be to use these new techniques for a systematic investigation of rainbows caused by realistically shaped rain drops."

###

Jensen, Sadeghi, Laven and their colleagues plan to present their findings at the SIGGRAPH conference in 2012, which will take place in Los Angeles. Jensen also plans to attend the next Light and Color in Nature conference, which will take place in Alaska. Will he try to simulate the Northern Lights next? He just might, he said.

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AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert! system.

Public release date: 6-Dec-2011
[ | E-mail | Share ]

Contact: Ioana Patringenaru
ipatrin@ucsd.edu
858-822-0899
University of California - San Diego

Computer scientists at UC San Diego, who set out to simulate all rainbows found in nature, wound up answering questions about the physics of rainbows as well. The scientists recreated a wide variety of rainbows primary rainbows, secondary rainbows, redbows that form at sunset and cloudbows that form on foggy days by using an improved method for simulating how light interacts with water drops of various shapes and sizes. Their new approach even yielded realistic simulations of difficult-to-replicate "twinned" rainbows that split their primary bow in two.

UC San Diego alumnus Iman Sadeghi, who did the work while a Ph.D. student at the Jacobs School of Engineering, his advisor, computer science professor Henrik Wann Jensen, and scientists from Spain, England and Switzerland, are set to publish their findings in ACM Transactions on Graphics in December of this year.

"This goes beyond computer graphics," Jensen said. "We now have an almost complete picture of how rainbows form."

Jensen is no stranger to advances in computer graphics. He earned an Academy Award in 2004 for research that brought life-like skin to animated characters. He has worked on a number of Hollywood blockbusters, including James Cameron's "Avatar."

Jensen, Sadeghi and colleagues originally set out to simulate rainbows to better understand how spherical water drops interact with light, resulting in the bright, multi-colored arcs that we are used to seeing when rain stops or in tropical, humid weather. They were hoping to improve techniques used in animated movies and video games.

"You usually don't get the opportunity to study such beautiful phenomena while working on your Ph.D. thesis," said Sadeghi, who is now a software engineer in the graphics division of Google in Santa Monica. "There is a lot more to rainbows than meets the eye."

As they started running various simulations, the scientists realized that the interaction of light with spherical drops could not explain some kinds of rainbows, such as twinned rainbows. Scientists turned to research showing that, as a water drop falls, air pressure flattens the bottom of it and shapes it like a burger. Jensen and his team called these slightly deformed water drops "burgeroids." "It's not a very mathematical term, but we like to use it," Jensen said. Simulations based on the so-called burgeroids, rather than on spherical drops of water, allowed the researchers to replicate a wide range of rainbows found in nature. "We are the first to present an accurate simulation of twinned rainbows," Sadeghi said.

The basic mechanism behind the formation of rainbows has been well understood for hundreds of years: A beam of light is both reflected and refracted within the water drop, and becomes strongly concentrated near the "rainbow angle" in the drop. The rainbow angle changes with the color of the light. As a result, sunlight separates into its spectral components, forming the colors we see in the sky. "The variation in the appearance of rainbows is due to the size and shape of rain drops" Sadeghi said.

It is surprising that the physics of rainbows are still not completely understood, Jensen said. In the past, eminent scientists, including Isaac Newton and French mathematician Rene Descartes, made calculations and conducted experiments to explain how rainbows form. But today, funding for rainbow research is scarce and so is work on the topic.

Jensen's quest to learn about the physics of rainbows led him to the Light and Color in Nature conference at St. Mary's College in St. Mary's City, Md. He served as keynote speaker and met Philip Laven, an internationally renowned expert on rainbows, who became one of the study's co-authors.

Until now, most simulations of rainbows had assumed that water drops are spherical, which isn't true for large rain drops, Laven said. In this paper, researchers have adopted a completely different approach and developed a more realistic model to recreate rainbows, he said.

"The simulations shown in this paper offer the prospect of a better understanding of real rainbows," Laven said. "I hope that the next step will be to use these new techniques for a systematic investigation of rainbows caused by realistically shaped rain drops."

###

Jensen, Sadeghi, Laven and their colleagues plan to present their findings at the SIGGRAPH conference in 2012, which will take place in Los Angeles. Jensen also plans to attend the next Light and Color in Nature conference, which will take place in Alaska. Will he try to simulate the Northern Lights next? He just might, he said.

---

[ | E-mail | Share ]

?

AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert! system.

Source: http://www.eurekalert.org/pub_releases/2011-12/uoc--css120611.php

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Tough Work Lies Ahead In Climate Talks

In Durban, South Africa, thousands of men and women poured into the streets in front of the International Conference Center, where United Nations talks about climate change are taking place. Host Audie Cornish speaks with NPR's Richard Harris, who is at the conference.

Copyright ? 2011 National Public Radio?. For personal, noncommercial use only. See Terms of Use. For other uses, prior permission required.

AUDIE CORNISH, HOST:

In Durban, South Africa, thousands of men and women poured into the streets in front of the International Conference Center there, where United Nations talks about climate change are taking place.

(SOUNDBITE OF WOMEN SINGING)

CORNISH: NPR's Richard Harris is at the talks and listened to speeches, songs and other appeals to diplomats to do more and act quickly in the face of a rapidly changing climate. He joins now from the conference center.

Hello there, Richard.

RICHARD HARRIS, BYLINE: Hi, Audie. How are you?

CORNISH: Good. Richard, this weekend marks the midpoint of the talks. Can you give us an update on the status?

HARRIS: Sure. Well, the first week of the talks is generally taken over by the lower-level negotiators. Let's remember what they are trying to do is come up with an agreement of some sort, to help slow the pace of global warming - an enormous task. The negotiators who've been here this past week have been working on text, and sort of trying to get the ducks in a row.

What happens over this weekend is the chief negotiators show up, the heads of the delegation, and they have usually more maneuvering room from their governments to actually cut deals.

CORNISH: Once those diplomats get there, what are the points of contention at the moment that they're going to have to take on?

HARRIS: Well, the two biggest are really what the future of these negotiations will look like and money. As for the future of the talks, the key part of the Kyoto Protocol, which was enacted in 1997 - was negotiated, will expire at the end of next year. And the big question is what will take its place? Will there be a second commitment period under this treaty, or will there be something else that will be substituted for it?

And that's a very, very delicate point here because lots of people have pretty well given up on the Kyoto Treaty. The United States never signed on to it. China has the obligations under it. Canada, Japan and Russia have all said we're done with this. And Europe is still hanging on to it a little bit. And the developing world is really, really wants to see it go forward, but that's not really looking like that's going to happen.

So, the backup is what will it do you look like after Kyoto, and can we find something that will do the job?

CORNISH: And to go back to something you said earlier, Richard. You mentioned money. What's the issue there?

HARRIS: Well, the issue is how the developed world, the rich nations of the world are going to help the developing world. And at a similar meeting last year in Cancun, negotiators agreed to set up something called the Green Climate Fund which would ultimately funnel about $100 billion a year; taking it from the wealthy nations to poor ones to help them adapt to climate change and to develop cleaner sources of energy.

Of course, the details of that fund are very contentious. They've been arguing over language for a whole year and they don't really have an agreement yet, except in principle, that that fund should exist. So, those issues are being hashed out here, and that's of obviously of great concern.

By the way, it's not just government aid money were talking about. It would also be money from private industry, as well. So, $100 billion here is a lot of money but we're not talking about foreign aid here.

CORNISH: Richard, given what you've said about the various degrees of commitment that different nations have to this issue, how should people measure success from a meeting like the one in Durban?

HARRIS: Well, I guess success, in some cases, is the avoidance of utter catastrophe, which is actually a potential outcome of this meeting. But let's start from the most optimistic scenario, that people who really want to see new promises for action, new sort of targets for emissions cuts before 2020. They're still holding on some hope although they will almost certainly be disappointed.

The U.S. officials have said the goals were set in Copenhagen and ratified in Cancun are good enough to get us to 2020 and they're not interested in notching those up at all. So that's probably not going to see major motion. But the question is maybe there could be some framework, some idea of what a treaty would look like post-2020.

CORNISH: And lastly, Richard, the climate talks switch cities every year. I would think it gives it a different flavor. So what's the atmosphere like in Durban?

HARRIS: It is summer here. It's kind of warm and rainy, and people are wearing short sleeves. It gives it kind of a casual feel. Walking around, people are still pretty casual about not only in clothing, but in attitude about how things are going so far. And they know that, you know, the tough work really lies ahead. The suits will go on, but ties will get tightened, et cetera.

CORNISH: NPR's Richard Harris at the United Nations climate talks in Durban, South Africa.

Richard, thanks so much.

HARRIS: Sure.

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Source: http://www.npr.org/2011/12/04/143107400/tough-work-lies-ahead-in-climate-talks?ft=1&f=1007

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