American Journal of Physics
A zero-order rainbow angle may be defined as the relative minimum angle of deviation of geometrical light rays transmitted without internal reflections through a transparent particle. If the incident rays are parallel and the particle is a sphere, such a minimum does not exist. But if the incident rays ale not parallel or if the particle has an elliptical rather than circular cross section, an angle of minimum deviation, hence a zero-order rainbow, can occur. For a spherical water droplet, the zero-order rainbow will occur when a point source is placed less than a droplet radius away from its surface. If a column of water with an elliptical cross section is illuminated by a plane wave, a zero-order rainbow will occur if the length of the major axis of the cross section is more than twice the length of the minor axis.
Lock, James A. and McCollum, Timothy A., "Further Thoughts on Newton's Zero-Order Rainbow" (1994). Physics Faculty Publications. 25.
Lock, James A. and T. A. McCollum. "Further Thoughts on Newton's Zero-Order Rainbow." American Journal of Physics 62 (1994): 1082-1089.
Copyright 1994 American Association of Physics Teachers. The article appeared in American Journal of Physics 62 (1994): 1082-1089 and may be found at http://aapt.scitation.org/doi/10.1119/1.17665