Supernumerary bows

Moir patterns occur in the most diverse phenomena. Two examples are mentioned here. It has been observed that only certain drop sizes cause supernumerary bows in a rain shower. The interference pattern produced by the rain wave front folding over on itself can be simulated with Moird patterns. 

Figure 8-43. Moire patterns simulate the formation of supernumerary bows in rain showers. The larger drops produce closely spaced bows at the top, while at the bottom the smaller drops produce more widely spaced bows. After Fraser [8-29], reproduced with permission.

Descartes analysis was a remarkable feat, but was deficient in two main respects. It said nothing about the colors of the rainbow nor about the supernumerary rainbows which sometimes appear faintly in the interior of the primary bow and on the exterior of the secondary bow. Newton resolved the problem of the colors,but the supernumerary rainbows are a wave interference effect and their explanation came more than 100 years later. 

---