RAINBOW. The rainbow is the best known of all optical meteorological phenomena, consisting of a colored arch formed opposite the sun on falling raindrops, and visible when ever the necessary conditions of a passing shower on one side and a clear and not too high sun on the other occur. Two bows are frequently seen, each exhibiting the full spec trum of colors from red to violet; hut in the inner or primary bow the red is the outer edge and violet the inner, while in the outer or secondary bow the order is reversed, the red being inside and the violet on the exterior. The colors are always arranged in a definite order, that of the solar spectrum — namely, red, orange, yellow, green, blue, indigo and violet, but shade imperceptibly into each other. The cause of this breaking up of the sunlight into its constituent colors is explained in most physical and meteorological textbooks,. but may be briefly summarized as follows: For the primary bow (Fig. 1), let PQR rep resent the section of a raindrop, and SF a ray of light falling on it. The .ray enters the drop at P, meets the surface again at R, is reflected to Q, where it leaves the drop in the direction of QE. The ray is refracted or bent on entering the drop at P and again on emerging at Q the amount of this refraction depending on the acuteness of the angle at which the ray meets the surface. Now it may be shown that there
is a particular point P, such that any ray from S striking the surface below P emerges again above Q, and any ray above P also emerges above Q — the former owing to the more acute angle of the reflection, and the latter to the greater refraction on entering and leaving the drop. The course of two such rays is shown by the dotted lines in Fig. 1. Q is thus a turn ing-point in the emerging rays, and near it a EQ more nearly vertical, and, therefore, the vio let rays form the outer edge and the red the inner of the secondary bow. The radius of the red is 50° 5', and of the violet 54° 0'. The space between the bows gets no reflected light, but that inside the primary and outside the sec ondary is faintly illuminated by rays such as are indicated by the dotted lines in Fig. 1 and their equivalents in Fig. 2, which are not shown. These rays with each other, and cause alternations of color which appear as spurious bows inside the primary and outside the secondary. They can only be seen with strong sunlight and small drops of rain.
The radius of the primary bow being roughly 40°, it is evident that it cannot be seen when the sun is at a greater elevation than this, as the highest part of the bow would lie below the horizon. See LIGHT.