What causes double rainbows?
A5: Figure 6-8 shows the path of a light ray (ray M) that contributes to the outer (or secondary) rainbow. This ray is reflected twice within the drop. Because each internal reflection is paired with a refraction of light out of the drop, less light is available to form the secondary bow, and thus it is less intense than the primary. At minimum deviation, this secondary-rainbow light is bent through an angle of about 231°, which places the secondary rainbow 51° from the antisolar point (51°= 231°-180°). Because the secondary rainbow is inherently dimmer than the primary, it may not always be visible. However, if the primary rainbow is very bright, look for its fainter secondary companion 9° outside it (Fig. 4-3). Q6: Why are rainbows often incomplete? A6: Since the raindrops are falling, their supply must be uninterrupted if the bow is to last. Because the edge of a rain shaft can pass quickly across the position where the rainbow might occur, the bow can appear or disappear rapidly. A
