Photograph by Ivy Turner.
What’s nice about this picture, besides that it is a nice picture, is that it shows a bright sky inside the primary rainbow and outside the secondary rainbow. These occur because (for the primary bow) droplets of water reflect light at all angles inside the primary bow, but the rays bunch up at the angle of the bow and give a bright dividing line between inside and outside; you may see a nice, if complicated, drawing here. Rays are likewise reflected at all angles outside the secondary bow but bunch up at the angle of the bow. The bows are colored because the index of refraction of the water droplets varies with color, but that is another subject.
P.S. Don’t forget the Photography Contest; entries close on Wednesday.