Band Between the Bows
September 30, 2002
On the above photo, in between the primary rainbow (green arrows) and the secondary rainbow (blue arrors), if you look closely, you should notice that the area of the sky is darker than the area on either side of these bows. This area is called Alexander's Dark Band. By the way, the Alexander of Alexander's Dark band is the Greek philosopher Alexander of Aphrodisias, who described the phenomenon in the early 3rd Century. Unfortunately, because he spent most of his time critiquing Aristotle, he never figured out if the dark band was the place where troubles melt like lemondrops.
When sunlight is backscattered to the eye via small spherical objects like cloud droplets, those rays that reflect once within the droplet (sun-bend-bounce-bend-eyeball) exit at angles up to about 42 degrees from the line from source (the sun) to the target (the droplet), with most of the light focused near this upper limit (this makes the primary bow). Those rays that reflect twice within the droplet (sun-bend-bounce-bounce-bend-eyeball) exit at angles greater than (but focused at) about 50 degrees. The area between these two arcs is devoid of such backscattered illumination and is darker.