Question
Wed April 04, 2012 By:
 

How is a secondary rainbow formed?

Expert Reply
Wed April 04, 2012
Secondary rainbows are formed by double internal reflection. Light is reflected twice from the inner surface of the raindrop before leaving the raindrop. We see rainbows because of the geometry of raindrops. When the sun shines from behind us into the rain, incident rays of light enter the drop and are refracted inwards. They are reflected from the back surface of the raindrop, and refracted again as they exit the raindrop and return to our eyes. Refraction is responsible for splitting the sunlight into its component colors.The size of the raindrops does not affect the geometry of the rainbow, although very tiny drops, such as those in fog or mist, reduce the effect. In this case, the effect of scattering overpowers the dispersive refraction effect. A "fogbow" has the arc of a rainbow, but appears as a bright white bow without spectral colors.
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