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Color me puzzled.

When I view a very bright rainbow, I see not only the typical secondary (color-inverted) bow but also a series of tightly spaced green and violet arcs on the lower (blue) edge of the primary bow. Are my eyes reacting to photon frequencies that are multiples of the green and violet frequencies? --David Thorman, Albuquerque, NM


You've accurately described the appearance of "supernumerary bows," but not their cause. (Frequency multiples would be outside the range of human vision, which spans just one octave of the light spectrum.)

The primary rainbow is produced by light that undergoes a single reflection inside each raindrop before emerging. The weaker, secondary bow is light that has made two internal reflections. But there's an infinite number of other possible (though less likely) paths through the drop, and some of these, like the primary bow, involve just a single reflection. It is these rays that produce the supernumerary bows. Robert Greenler, in his book Rainbows, Halos, and Glories (Cambridge University Press, 1980), explains how they are formed by constructive interference among pairs of such rays that differ in path length by one wavelength, two wavelengths, and so on.

In Light and Color in the Outdoors (Springer-Verlag, 1993), Belgian astronomer Marcel Minnaert tells how you can count the supernumerary bows and look at their spacing to gauge the size of the raindrops producing them.

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Title Annotation:hobby Q&A; on watching a rainbow
Author:Sinnott, Roger W.
Publication:Sky & Telescope
Article Type:Brief article
Geographic Code:1USA
Date:Sep 1, 2007
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