Alexander’s band, also known as Alexander’s dark band, is an optical phenomenon observed in rainbows. It refers to the region between the primary and secondary bows, which often appears noticeably darker to an observer compared to the rest of the sky.
The areas of the sky around a rainbow may appear blue or grey depending on weather conditions and cloud cover. However, these areas outside, inside, and between the primary and secondary rainbows tend to have distinct tonal differences from one another:
The area inside the arcs of a primary rainbow always appears tonally lighter than the surrounding sky.
The area outside primary and secondary rainbows appears darker.
The area between primary and secondary rainbows appears the darkest – this is Alexander’s band.
Alexander’s band can be explained by the fact that fewer photons are directed from this specific area of the sky toward an observer.