Alexander’s band (Alexander’s dark band) is an optical effect associated with rainbows. The term refers to the area between primary and secondary bows that often appears to be noticeably darker to an observer than the rest of the sky.
- The areas of sky around a rainbow may appear blue or grey depending on weather conditions and the amount of cloud in the sky. But these areas outside, inside and between primary and secondary rainbows tend to appear tonally different from one another:
- The area inside the arcs of a primary rainbow always appears tonally lighter than the rest of the 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 area of the sky toward an observer.
- The raindrops that form a primary rainbow all direct exiting light downwards towards an observer so away from Alexander’s band.
- The raindrops that form a secondary bow all direct light upwards, so away from Alexander’s band, before a second internal reflection directs light downwards towards an observer.
- Alexander’s band is named after Alexander of Aphrodisias, an ancient Greek philosopher who commented on the effect in his writing.