Photographing a natural fourth-order rainbow
Has a fourth-order rainbow, formed by four internal reflections inside raindrops, ever been captured in a natural photograph after more than a century of failed attempts?
History and references
Settled in 2011 by Michael Theusner.
Like the third-order rainbow, the fourth-order (quaternary) rainbow was predicted by 19th-century optical theory but sits near the sun, making it exceptionally hard to see or photograph against sky glare. Just a month after Großmann's first photograph of the third-order rainbow in April 2011, German physicist and photographer Michael Theusner independently photographed and, with image processing, extracted a natural fourth-order rainbow for the first time. His result and analysis were published in the same 2011 Applied Optics special issue devoted to higher-order rainbows. Together the two results closed a roughly 150-year gap between theoretical prediction and observational confirmation for third- and fourth-order rainbows.
Within weeks of the first-ever photograph of a third-order rainbow, a fourth-order one was captured too, finally putting two centuries of optical theory to direct observational test.
Explanations that circulated
- As with the third-order bow, it was widely assumed a fourth-order rainbow was effectively unobservable in nature rather than merely extremely faint and positioned close to the sun's glare
References
- Photographic observation of a natural fourth-order rainbow, Michael Theusner, Applied Optics, 2011
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