White rainbow

Photograph by Paul Strode.

This splendid picture by Paul Strode shows a white rainbow, or a fog bow. The white rainbow is seen when the droplets are very small, as in a fog. In that case the angle of diffraction is at least much as the angle of refraction, so the colors overlap, and the rainbow appears white. M. Minnaert, in The nature of light and color in the open air (§128), notes that the width of the white rainbow may be as much as twice the width of the ordinary rainbow. Additionally, the colors may be reversed, because red light is diffracted more than blue, whereas it is refracted less. In Dr. Strode's picture, we see a "conventional" rainbow at the left merging into a white arc where, presumably, the droplets are smaller. The arc is about the same width as the colored section, which hints that the droplets are sized so that diffraction barely covers refraction. In consequence, we do not see any color, reversed or otherwise.

Incidentally, contrary to popular descriptions here and (especially) there, the light is always refracted by the droplets, but sometimes diffraction plays a role as well.