Normal atmospheric dispersion as the cause of the "green flash" at sunset, with illustrative experiments

Abstract

The phenomenon of the "green flash" or "green ray" at sunset has often been discussed. The reader may be reminded that this effect is sometimes seen just as the sun’s upper limb vanishes below the horizon; for the last 2 or 3 seconds, the vanishing upper limb appears of a more or less vivid green colour. The most obviously necessary condition is a clear sky at the horizon, a condition much more often fulfilled in the tropics than in this country. For a more detailed description reference may be made to a paper by A. A. Rambaut, and some other authors will be referred to in the sequel. It is not necessary to go back to the earlier literature. The theory which attributes the green sensation to retinal fatigue, though still sometimes met with, is conclusively disposed of by the simple observation that the green flash occurs at sunrise as well as at sunset. It is now generally recognised that the correct explanation depends on atmospheric refraction. Refraction keeps the sun’s upper limb in view when its position, geometrically considered, is below the observer’s horizon. Refraction is accompanied by dispersion, though the latter effect is comparatively small, and therefore the more refrangible part of the spectrum of the sun’s upper limb should be the last to disappear. The green usually seen is in fact more refrangible than the general light of the disc at the horizon.