FURTHER THOUGHTS ON NEWTONS ZERO-ORDER RAINBOW

A zero-order rainbow angle may be defined as the relative minimum angl e of deviation of geometrical light rays transmitted without internal reflections through a transparent particle. If the incident rays are p arallel and the particle is a sphere, such a minimum does not exist. B ut if the incident rays ale not parallel or if the particle has an ell iptical rather than circular cross section, an angle of minimum deviat ion, hence a zero-order rainbow, can occur. For a spherical water drop let, the zero-order rainbow will occur when a point source is placed l ess 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 zer o-order rainbow will occur if the length of the major axis of the cros s section is more than twice the length of the minor axis.