The degree of polarization of rainbows of order k with k >= 3 is bounded in the interval [75%, 78%], where 75% is the limit for k -> infinity. A polarization filter can improve the signal-to-background ratio of the third and fourth rainbows by a factor of 2, which may lift their visibilities in natural circumstances above the threshold of human visual perception. Under optimal circumstances, the latter may be true for the recently photographed green fingerprint of the fifth rainbow, even without the aid of a polarization filter. The prospects for observing the sixth rainbow are unclear. There exists a possibility that the signal of the natural seventh rainbow (appearing at 64 degrees from the Sun) may be separated from its background if photographed under perfect conditions through a polarization filter. (C) 2014 Optical Society of America
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