rainbow

   A term used to denote the well-known coloured arc sometimes seen against a sunlit background of falling water drops. Rainbows are typically seen when the Sun is low in the sky, and the observer is facing a raincloud on the horizon opposite the Sun (i.e. at the antisolar point). When more than one rainbow is visible (i.e. a double rainbow), the inner, brighter one is called the primary bow and the outer, fainter one the secondary bow. Primary rainbows are coloured blue to red outwards from the antisolar point, whereas the colours of secondary rainbows are reversed, i.e. red to blue outwards from the antisolar point. The relatively dark area in between the primary and secondary bows is called Alexander's band or Alexander's dark band, after the Greek philosopher Alexander of Aphrodisias (c. AD 200). Sometimes a third or 'supernumerary' bow can be seen inside the primary bow or outside the secondary bow. Supernumerary bows tend to be even fainter than secondary bows. Still fainter bows, such as quaternary bows, can be produced experimentally by the laser illumination of single droplets. It is believed that they also occur in nature but that they are seldom observed because of their extremely low visibility. The colours of all rainbows extend beyond the visible colours towards infrared and ultraviolet. The brightness of the colours perceived depends on various factors, such as the strength and hue of the sunlight, the size of the water drops, the number of reflections inside the water drops, and the colour of the background against which the rainbow is seen. The brightest result is obtained with low, bright sunlight, relatively large water drops, a single internal reflection, and a dark-coloured background. Reddish sunlight, seen around the moment of sunset or dawn, may produce a red rainbow. When a rainbow is produced by moonlight, the term lunar bow is used. Lunar bows tend to be colourless. They should not be confused with the white rainbow or *Ulloa circle. Other types of rainbow include the surf bow (seen in the spray of crashing waves at the beach), the swimmer's bow (seen by swimmers at less than a metre away in the spray they produce), the road spray bow (seen in spattering water from puddles on the road), the marine bow (seen at the prow of a ship), the reflection rainbow (created by sunlight that is first reflected in water), the garden hose bow, the geyser bow, and the mist bow. All types of rainbow are classified as * physical illusions. They are attributed to the refraction and reflection of sunlight by water drops. Because of their lack of a tangible substratum in the extracorporeal world, they are also referred to as fiction illusions.

   References

   Lee, R L , Fraser, A B (2001) The rainbow bridge. Rainbows in art, myth, and science. University Park, PA: Pennsylvania State University Press.

   Lynch, D.K., Livingston, W. (1995). Color and light in nature. Cambridge: Cambridge University Press.