Alexander’s band

Alexander’s band (Alexander’s dark band) is an optical effect associated with rainbows. It 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.

About Alexander’s band
  • Alexander’s band is named after Alexander of Aphrodisias, an ancient Greek philosopher who commented on the effect in his writing.
  • The darker area between primary and secondary rainbows can be explained by the fact that fewer photons are directed from that 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.
  • Plenty of light is scattered into the area between primary and secondary rainbows but very little of it travels towards an observer.