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Double Bows
Thu Jul 22, 2004
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 centre 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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