Stories from Physics for 11-14 14-16
In around AD 200, Alexander of Aphrodisius observed that, during rain, the area between primary and secondary rainbows appears considerably darker than the surrounding sky. This area has been labelled Alexander’s band. The phenomenon occurs because the refractive index of light means that light from raindrops in the region of the sky between the two rainbows cannot reach the observer.
G. P. Können, Polarized Light in Nature, Cambridge, Cambridge University Press, 2008, p. 49.