Why are bright rainbows most vivid near their base?
A11: As explained in Chapter 9, large raindrops flatten as they fall but smaller ones do not. As a result, all sizes of raindrops contribute to the rainbow near its base, and large drops make this part of the bow both bright and colorful. However, as we approach the top of the arch only small drops contribute to the rainbow, and these yield a less intense, more pastel bow. See Fig. 7-21 for an example of this color and brightness gradation. Q12: Why is the sky dark between the primary and secondary rainbows? A12: By definition, raindrops between the bows cannot send you any light that contributes to either the primary or secondary. In other words, light that has been internally reflected once (the primary) or twice (the secondary) by water drops does not reach you from this part of the sky, so the sky looks comparatively dark there. This dark band is known as Alexander s dark band (Chapter 4) and is most evident if the primary and secondary bows are bright (Fig. 9-2 and Minnaert 1993,
