The region between a primary and secondary rainbows seems to be darker.Can someone explain this phenomena?

Question

The region between a primary and secondary rainbows seems to be darker.Can someone explain this phenomena?

Answer 1

Hi Ali,

     The primary rainbow has a radius of 42°, there sometimes is also a secondary rainbow visible which has a radius of 51° around the antisolar point. Its sequence of colors is reversed in comparison with the primary rainbow. Sometimes one can see that the area inside the primary rainbow is brighter than the area between primary and secondary rainbow. The dark area is also called "Alexander's Dark Band". This name derives from Alexander of Aphrodisias, who lived around the year 20 Ca.C. and was a philosopher and commentator of Aristoteles.  The optical explanation is that outside the secondary bow, there is still overlapping scattering similar to inside the primary bow. Between bows, no light emerges from the raindrop. Thus, between bows the sky is darker.  This lack of scattered light forms Alexander's Dark Band.

Answer 2

Light reflecting off the rain drops is dispersed to the side at varying angles to make the rainbow's colors.  This light has to come from somewhere and it comes from the light inside the primary rainbow and outside the secondary - between the two.  It actually happens inside the primary all the time, but we don't notice it as much because if we can't see the secondary, it is much harder to judge what is going on.