The secondary rainbow scattering angle for spheroidal drops of water is virtually independent of aspect ratio for most visible wavelengths. For most solar heights the residual aspect-ratio dependence shifts the bow toward a smaller deviation angle if the drop size increases. These two facts explain why the supernumeraries of the secondary rainbow are never seen in rain showers. At high solar elevations the flattening of drops results in a shift of the secondary rainbow toward a larger deviation angle. It is shown that this shift is still large enough to cause the formation of the first supernumerary in red light. This red supernumerary of the secondary rainbow may be observable by eye in natural showers if a red filter is used to remove the obscuring contribution of shorter wavelengths to the light of the rainbow. For indices of refraction far from that of water, a strong aspect-ratio dependence of the secondary rainbow angle is shown to be present. Some possible implications of this for the formation of a hyperbolic umbilic diffraction catastrophe in the secondary rainbow pattern are indicated.
© 1987 Optical Society of America
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