Some Aspects of the Optics of the Rainbow and the Physics of Rain

It has hitherto usually been believed that the classic rainbow theories of Descartes and Airy for spherical drops Completely satisfy the observations. However the well-known deviations of large falling drops from a spherical shape require changes in the rainbow angle, especially for scattering in the vertical cross section of the drop. Flattened raindrops of different radii cannot form a perceptible bow; this may explain why the interference bows are more visible at the top of the bow than towards the base, although their spacings do not change. Preliminary investigations on the oscillations of large falling raindrops have been made, Such oscillations can result in blurring or washing out the rainbow. Rainbow intensities for measured drop size distributions have been calculated from the Airy theory, Since drops with radii larger than 0.2 to 0.5 mm are flattened and oscillating, they contribute little to the rainbow. Analysis of the rainbow can therefore only give information about the spectrum of the smaller drops.