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article imageEssential Science: Why some rainbows are completely red

By Tim Sandle     Dec 21, 2015 in Science
Rainbows containing only a limited fraction of the light spectrum can form. This happens as the sun drops lower in the sky. New research has found the presence of rainbows that are primarily red.
At school we learn the colors of the rainbow by using the acronym ROYGBIV or the mnemonic: “Richard Of York Gave Battle In Vain” (both of which represent the Newtonian spectrum: red, orange, yellow, green, blue, indigo and violet.) If we’re lucky we can make out all seven colors of the visible spectrum, although it’s a bit of an eye-strain to distinguish between indigo and violet. There are probably more than seven colors; the fact that we can only see seven is based on the limitations of human perception. Moreover, there’s a debate within physics about the worthiness of indigo — is it a separate color or simply "deep blue"?
UC San Diego scientists studied rare primary twinned rainbows with computer simulations.
UC San Diego scientists studied rare primary twinned rainbows with computer simulations.
Photo courtesy of Jacobs School of Engineering
It seems that the kindergarten chant through the spectrum is not always accurate for it is possible to have an "all red" rainbow.
A rainbow is a type of meteorological phenomenon that sometimes occurs after rain. The natural light show is the result of various physical phenomena: reflection, refraction and dispersion of light within water droplets. These three processes lead to a spectrum of light appearing in the sky for a short period of time. To the human observer, the colors are seen by light refracted as it enters a droplet of water, then reflected inside on the back of the water droplet, and then refracted again as the light leaves the droplet. In a sense, a rainbow is an optical illusion: it doesn’t physically exist and it can only be ‘seen’ 42 degrees from the direction opposite to the light source.
Not all colorful light patterns are rainbows. For example, when sunlight passes through ice crystals, halos form and they are mostly colored. At other times, small cloud or fog droplets diffract light to form ringed glories, coronae and the jumbled colors of iridescent clouds.
Rainbow with lightning over Kawana Island.
Rainbow with lightning over Kawana Island.
thinboyfatter / Flickr
The size and shape of rainbows vary. Some can be complete circles (although these are very rare); most often rainbows appear as arcs. At odd times a double rainbow is seen. Here, the order of colors is different and the two rainbows are separated by what is termed the "Alexander band." With one rainbow, red light normally appears on the outside and violet on the inside; with a second rainbow, the colors are reversed so that red appears on the inner part of the arc and violet on the outer part. Third and fourth and even multiple bows can sometimes be seen. So, occasionally, can what are called “supernumerary” bows. Here extra bands of green and violet occur, caused by complex interference patterns.
At other times, new research reveals, rainbows don’t show all of the colors at all. A variation at play, that shapes the human perception of a rainbow, is linked to the height of the sun above the horizon. Changes with the sun result in arcs that contain only a fraction of the traditional ROYGBIV. This was recently reported at the American Geophysical Union’s fall meeting, which took place in December 2015.
According to Science News, sometimes a rainbow will contain just part of the color spectrum and some can even be monochromatic rainbows, with the most common variant being "all red." This is due to the size of the light-bending water droplets.
These rare events have been cataloged by atmospheric scientist Jean Ricard of the National Center for Meteorological Research in Toulouse, France. The scientists examined hundreds of photographs of rainbows. Through this analysis, they found the sun’s position plays a far more significant role in shaping the color of rainbows than any previous research had shown. In particular, at sunset, when the sun is at a low point in the sky, this means light is required to travel a longer distance through the atmosphere. As light passes, the air scatters away other colors leaving reddish light. The light remaining thickens a rainbow’s red band, which ‘squeezes out’ the orange color band and leaves a red-only rainbow.
Horseshoe Falls and Niagara State Park  New York  with rainbow present viewed from across the river ...
Horseshoe Falls and Niagara State Park, New York, with rainbow present viewed from across the river on River Road, Niagara Falls, Canada.
Speaking with the Daily Mail, Jean Ricard, said that few rainbows look alike: “because when we look at a rainbow, one second later, the drops which form the primary bow and the secondary bow are not the same, because they are falling.”
It would seem that rainbows are not always quite how they appear.
This article is part of Digital Journal's Essential Science series. Other articles in the series are "Space-food for astronauts made from bacteria"; "Health effects of antibiotic use"; "Graphene makes improved night vision tech"; "Personalized medicines, the health innovation”; and “Power paper can store electricity.”
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