Atmospheric
Optics
Samos Supernumeraries ~ Primary and secondary rainbows arch over the Greek island of Samos. Both are fringed by supernumerary bows, . Images by Manolis Thravalos (more images).
Images ©Manolis Thravalos, shown with permission
Greens and purples dominate the supernumeraries. Each is a mix of all the spectral colours.
Right: Levels enhanced crop from the top image. The primary is overexposed. Two secondary supernumeraries show. They are broader and more widely spaced than those inside the primary.
Far right: A colour subtraction enhancement. Two, possibly three supernumeraries. Spacing decreases farther from the main bow.
The primary comes from rays reflected once inside raindrops.
There are always two rays that emerge at the same angle. Only at the rainbow's rim do they coincide.
Rays passing closer to the drop centre deflect inside the rainbow rim and light the sky there. Each ray is brightly coloured but the colours from millions of drops mix to white.
Two rays deflect through the same angle inside the rainbow's rim.
Not straight line rays but waves. Waves overlap. Where crests coincide there is light. Where they cancel there is darkness.
The overlap depends on angle. There are a series of bright fringes, their angle colour dependent - supernumeraries.
Supernumeraries extend to the bow centre. They get fainter and crowd ever closer together.
These bows have all. Almost semicircles from the low sun. Nearby glinting raindrops extend the primary beneath the horizon. The sky whitens inside the primary and the nearby Turkish coast blots out.
The same number of raindrops are there as elsewhere but they glint light, rainbow light, to the eye and camera. Between the bows the drops glint no light towards us (yet they glint elsewhere) – Alexander’s Dark Band. The sky lightens again outside the secondary from more rainbow light. Rainbows are not rings.
The inner primary and outer secondary light creates supernumeraries. Those inside primaries are common.
Secondary supernumeraries - seen at 2 o’clock on the top image and enhanced at right – are rare. They are faint. They are broad. They have low contrast. A dark background sky helps visibility. So does fine rain whose drops are nearly the same size.
