In the refraction phenomena that we have thus far considered the air is the medium in which the light rays are bent and distorted. In the production of the rainbow, light undergoes refraction, dispersion (separation of the spectral colors) and reflection by passing through drops of water in the atmosphere; especially falling raindrops.
The rainbow, perhaps because it is such a common sight, is seldom observed with careful attention. Hence few people realize that there are many varieties of this beautiful meteor, and various erroneous ideas about it are prevalent. The rainbow is always seen in the part of the sky opposite the sun—or the moon, in the case of the lunar rainbow—and is high in the heavens when the luminary is low, and low when the luminary is high. Generally less than a semicircle of the bow is visible, and never more, except from an eminence. (Aeronauts occasionally see a complete circle.) The outer border of the bow is red and the inner blue or violet. Contrary to popular belief and to statements sometimes found in reference books, it is almost never possible to distinguish all seven of the spectral colors in a rainbow; four or five is the usual limit.
The ordinary or primary rainbow has a radius of about 42 degrees at its outer edge. Very commonly a secondary rainbow is seen, concentric with the primary bow, and having a radius of about 50 degrees. The secondary is fainter than the primary, and its colors are in opposite order—red inside and violet outside. Additional bands of color, chiefly red and green, may often be detected adjacent to the inner edge of the primary bow and, less frequently, along the outer edge of the secondary bow. These are known as supernumerary bows. The space between the primary and secondary bows is somewhat darker than the rest of the sky.
The common rainbows differ much among themselves in the number and purity of their colors, the width of the bows, etc., these differences depending especially on the size of the raindrops. The minute drops of a fog sometimes give rise to a bow that is almost devoid of color—the “white rainbow,” or “fog bow.” The rainbows produced by the moon commonly show little color, on account of the relative faintness of the light, but the brighter lunar rainbows are often very distinctly colored.
INTERSECTING RAINBOWS
(After a sketch by T. Hodge. Courtesy of Scientific American.)
Reflected rainbows are sometimes seen upon a sheet of water; and again the image of the sun, as reflected by such a surface, may give rise to both primary and secondary rainbows in the sky, which appear to intersect those produced by the sun directly. A horizontal layer of water drops below the level of the observer’s eye occasionally produces the so-called horizontal rainbow. This may be formed over a bedewed field or other surface (the “dew bow”); or the drops may be those of a low-lying sheet of fog, or of water deposited on a floating film of oil, or, finally, actual raindrops, seen from an elevation, such as the summit of a mountain. Horizontal rainbows formed by rain have been seen from the Eiffel Tower.
The common saying,
A rainbow in the morning
Is the shepherd’s warning;
A rainbow at night
Is the shepherd’s delight,