The New Student's Reference Work/Rainbow

Rain′bow′, an optical phenomenon of great beauty appearing in the sky when rain is falling from clouds opposite the sun. It consists of a number of concentric arcs whose successive colors are arranged in the same order as the colors of the solar spectrum. There usually are two bows: an upper and a lower. The lower one is always the brighter and is called the primary bow; the upper is often very faint, and is called the secondary bow. In the primary bow the colors begin with red at the top and end with violet at the bottom; in the secondary bow the order is just reversed.

The explanation of the rainbow was first given by Antonio de Dominis, archbishop of Spalatro, about 1600 A. D., who showed that a glass sphere filled with water would reflect a parallel incident beam of light in such a way as to produce a complete rainbow on a screen surrounding the source of light; but this explanation was, of course, incomplete until Newton proved that color is not produced at the refracting surface but is due to the decomposition of white light. This experiment of Dominis can be repeated by going into the Cave of the Winds at Niagara Falls where, on a bright day, the parallel incident beam and the complete rainbow of 360° are easily obtained, the particles of spray replacing the glass-bulb filled with water. The primary bow is produced by rays which have undergone one internal reflection; the secondary by rays which have suffered two internal reflections. The angular width of the primary bow is twice 42° 2′ for the red rays; and twice 40° 17′ for the violet rays. The corresponding angle for the red rays of the secondary bow is 50° 57′; and for the violet rays 54° 07′. For the details of the reflections and refractions here occurring see any good textbook of physics, as Watson's.