I simply love Atmospheric Optics for nearly everything about lights in the sky. Regarding secondary rainbows or “double rainbows” they say that the secondary is nearly always fainter than the primary, with colors reversed and more widely separated:
Light can be reflected more than once inside a raindrop. Rays escaping after two reflections make a secondary bow.
The secondary has a radius of 51º and lies some 9º outside the primary bow. It is broader, 1.8X the width of the primary, and its colours are reversed so that the reds of the two bows always face one another. The secondary has 43% of the total brightness of the primary but its surface brightness is lower than that because its light is spread over its greater angular extent. The primary and secondary are are concentric, sharing the antisolar point for a center.
About this particular rainbow from April 2, 2016, Gerry writes: “Double rainbow from the other night after the storms. The weather in Michigan can change quickly, from rainbows to snow. Yep, that’s Michigan.”
More rainbows on Michigan in Pictures.