October 6, 2011
Photos Capture Rare Triple Rainbow
Witnessing a double rainbow may seem like a sporadic event, and seeing three at once is extremely rare. But new photographs show this exceptional phenomenon in action.
Scientific reports of tertiary (three) rainbows are so rare that only five have ever been reported in the last 250 years. These optical phenomena, caused by three reflections of light rays within a raindrop, have been confirmed to exist thanks to photographic perseverance and a new meteorological model providing the scientific support to find them.
The work is published this week in the Optical Society´s (OSA) journal Applied Optics.
Raymond Lee, a professor of meteorology at the U.S. Naval Academy, predicted around a year ago how tertiary rainbows might appear and challenged rainbow hunters to go out and find them. And apparently some did so, as photographic evidence confirms the existence of not only a tertiary rainbow, but a quaternary (fourth) rainbow as well.
Although exceptionally rare, tertiary and quaternary rainbows are in fact possible, and are natural products of the combination of refraction, dispersion, and reflection inside raindrops. All rainbows are created through this process. Refraction occurs when sunlight bends as it moves from air into water and vice versa. Water droplets bend each of the colors in sunlight by a slightly different angle. This is called dispersion, and it separates the colors to create a rainbow.
A large amount of the multi-colored light passes through the other side of the raindrop, but some is reflected. This intense light is bright enough to create a visible primary rainbow. A double rainbow occurs because not all that light exit´s the raindrop. A certain amount is reflected back into the raindrop and goes through the entire process all over again. Although this light is dimmer, it is sometimes bright enough to produce a secondary rainbow just outside the first.
A third rainbow is produced when light is reflected once again into the raindrop, and this reflection is even dimmer than the secondary rainbow, and much harder to find because instead of forming away from the Sun, a tertiary rainbow forms around the Sun. To see it, you would have to look into the Sun´s glare.
This may be why only five confirmed scientific reports have occurred in the past two-and-half centuries.
Lee reviewed each description carefully and eliminated one questionable account. He found common elements in the other four events, all described as tertiary rainbows that appeared for a few seconds against a dark background of clouds about 40 degrees from a brightly shining sun.
Along with colleague Philip Laven, Lee used a mathematical formula to predict what conditions were needed to produce visible tertiary rainbows.
First, they needed dark thunderclouds and either a heavy downpour or a rainstorm with nearly uniformly sized droplets. Under these conditions, if the Sun broke through the clouds, it could project a tertiary rainbow against the dark clouds nearby. The contrasting colors would make the dim tertiary visible.
Lee´s findings sparked intense debates when he published them at last year´s International Conference on Atmospheric Optics. Some scientist´s insisted that past descriptions were wrong and that tertiary rainbows were too dim to see in the Sun´s glare.
Elmar Schmidt, an astronomer at Germany´s SRH University of Applied Sciences, took Lee´s challenge to find a tertiary rainbow, and alerted his fellow rainbow chasers as well. Since then, rainbow chasers Michael Grossman and Michael Theusner have taken photos of tertiary rainbows. One of the photos even shows a quaternary rainbow.
Both images appear in the Applied Optics publishing with Lee and Laven´s work. The images underwent only minimal image processing to improve the contrast under the challenging photographic conditions.
At first, Grossman believed he was only seeing a double rainbow. But as the rain intensified, he turned toward the Sun. “It is really exaggerated to say that I saw it, but there seemed to be something,” he said.
The pictures Grossman snapped in the rain were the first to show a tertiary rainbow.
“It was as exciting as finding a new species,” Lee added, referring to finding a tertiary.
Image 1: The third-order (tertiary) rainbow (left), accompanied by the fourth-order (quaternary) rainbow (right). They appear on the sunward side of the sky, at approximately 40° and 45°, respectively, from the Sun. This is the first picture ever of a quaternary rainbow in nature and the second picture ever of a tertiary rainbow. Credit: Michael Theusner/Applied Optics.
Image 2: Original image of a third-order (tertiary) rainbow. The image was taken May 15, 2011, in Kampfelbach, Germany and is the first-ever picture of a tertiary rainbow. Two reference positions (A and B) for image orientation are indicated. (b) Processed version of image (a) after contrast expansion and unsharp masking, showing a rainbow-like pattern next to the image center, marked by the arrows. Credit: Michael Grossmann/Applied Optics.
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