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Double Bows
Thu Jul 28, 2005
Listen in RealAudio 
Rainbows have long been part of human mythology from the story of Noah to Native American beliefs and Viking legends. They have also fascinated scientists through the ages, including folks like Aristotle and Isaac Newton. Hi, I’m Bryan Yeaton for The Weather Notebook.
To form rainbows, we need two things: suspended water drops and sunlight. Plus, the viewer needs to have the light source behind him or her. When sunlight strikes the falling drops, it is internally refracted, then reflected, and refracted again before returning back toward the sun. The refractions split the sunlight into a beam containing the well-known colour spectrum that emerges from the drop at an angle of around 42 degrees. This is the primary rainbow.
However, not all the light escapes the drop. Some rays are reflected internally a second time before leaving the drop at around 51 degrees. The resulting secondary rainbow lies outside the primary arch.
In the primary rainbow, the violet and blue end of the colour spectrum is on the inner edge of the arc, with red on the outside. In the secondary bow, the sequence is reversed. The secondary rainbow lies above the primary bow, and is about 43 percent less bright than the primary. Additional internal reflections can occur, but their resulting bows are usually too dim to be seen.
Between the two bows we see a much darker region known as Alexander’s Dark Band. Most of the returning light has been bent out of our view within this region of sky, leaving it somewhat depleted of light.
In contrast, the rainbow center appears much brighter than the surrounding sky because there the sunlight reflects directly back to us from the outer drop surfaces.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is supported by The National Science Foundation and Subaru of America.
Today's Links
What causes a rainbow?
http://www.sec.org.za/physics/p10rain.html
Secondary Rainbows
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/wtr/rnbw/scnd.rxml
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