Big Sky, Big Bows
August 18, 2005
Provided and copyright by: Joel White
Summary authors & editors: Jim Foster; Joel White
This gorgeous double rainbow occurred near Big Sky, Montana (June 29, 2005) as the Sun was setting and a rain shower was passing eastward. Rainbows are caused by sunlight passing through water droplets, and after a single reflection, in the case of a primary bow, emerging at an angle of approximately 42 degrees. The secondary rainbow is created when the Sun's light is twice reflected in raindrops, and the rays exit the drops at an angle of about 51 degrees.
In addition, not only are both the primary and secondary bows visible, but supernumeraries can also be detected on both the inside of the primary bow and the outside of the secondary bow. Supernumerary bows result from diffraction rather than simple reflection and refraction. Note also Alexander's Dark Band between the two bows. When a secondary rainbow is visible, in addition to the sky being brighter on the inside of the primary bow, it's also brighter to the outside of the secondary bow. Therefore, the area in between appears darker.
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