REFRACTION PHENOMENA

463

Entering and Emerging Rays.—Since a raindrop is spherical, it is obvious that its effect on incident radiation from the sun, or other spherical or point source, is symmetrical about an axis through the center of the drop and the luminous object. Hence, in the study of the rainbow, it is sufficient to use only a single plane containing this axis, tracing the rays incident over one quadrant of the intersection circle and noting the resulting phenomena. It is also obvious that, neglecting sky light, solar rays are parallel to within the angular diameter of the sun, 0.5°, about, and that as a first approximation they may be

FIG. 174.—Course of light through a raindrop and the corresponding wave fronts.

regarded as strictly parallel. Let, then, the plane of Fig. 174 pass through the centers of a raindrop and the sun, and let AB be the wave front of parallel rays incident, as shown, above the normal or axial ray (ray passing through center of drop). An equal amount of light clearly enters below the normal ray, but for simplicity this is omitted. Similarly, that portion reflected from the outer surface is ignored, as is also all that is internally reflected more than once. This reduces the problem of the rainbow to its simplest terms, but loses none of its generality, since additional internal reflections merely change angular dimensions and brightness. The heavy line shows the course of the Descartes ray, or ray of minimum deviation for light of air-water refractive index, y%. The courses of other rays are, approximately, as indicated. Since the deviations of the rays incident between the axial and the Descartes