What causes the green flash sometimes seen just as the sun sets?
Why does a brief flash of green light sometimes appear at the very top edge of the sun in the instant it disappears below a clear, distant horizon at sunset (or reappears at sunrise)?
History and references
Settled in 1954 by Established via atmospheric refraction/dispersion theory; laboratory-confirmed by Lord Rayleigh (early 20th c.) and quantified by D. J. K. O'Connell (1954, 1958).
Earth's atmosphere acts like a weak prism, refracting sunlight by an amount that depends on wavelength (dispersion), so that as the sun sets the image of its top limb is effectively a stack of slightly offset colored images, with the blue/green image refracted highest and lingering a fraction of a second longer than the red image after the rest of the disk has dipped below the horizon. In practice violet and blue are usually scattered away or absorbed by the long atmospheric path, leaving green as the last visible color, producing the brief flash; this refraction-and-dispersion explanation was demonstrated experimentally by Lord Rayleigh with an artificial light source and a matched prism, and put on firm quantitative footing by O'Connell's mid-1950s studies, which is essentially the explanation Walker's book reports.
A sunset's very last sliver of light is a natural prism experiment, sorting the sun's colors by how much the atmosphere bends each one.
Explanations that circulated
- The green flash is an optical illusion or afterimage produced by the eye, not a real atmospheric effect
- It is caused by ozone absorption of yellow/orange light alone, unrelated to refraction
References
- Normal atmospheric dispersion as the cause of the green flash at sunset, with illustrative experiments, D. J. K. O'Connell, Proceedings of the Royal Society A, 1930
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