Photographing a natural third-order rainbow

Settled in 2011 by Michael Großmann, Elmar Schmidt, Alexander Haußmann.

Descartes' 1637 geometric optics and later Airy's 19th-century wave theory both predicted a faint tertiary rainbow appearing near the sun itself (not opposite it, like the familiar primary and secondary bows), making it far harder to see against bright sky glare. For nearly 250 years reported sightings were unreliable and unphotographed, leading some to doubt it was observable in nature at all. In May 2011, amateur physicist Michael Großmann, guided by careful geometric prediction of exactly where and when to shield the sun, captured the first convincing photographs, later confirmed and analyzed by Schmidt and Haußmann in Applied Optics. The achievement closed a centuries-old observational gap between theory and evidence for higher-order rainbows.

A rainbow effect calculated by Descartes in 1637 wasn't actually seen and photographed in nature until 2011, once someone worked out precisely where to look with the sun blocked out.

Explanations that circulated

• It was long assumed the third-order rainbow could never be photographed because it forms on the sun-facing side of the sky, where forward-scattered sunlight was believed to always overwhelm it; careful sun-shielding and image processing showed it is merely extremely faint, not undetectable

References

• Photographic evidence for the third-order rainbow, Michael Großmann, Elmar Schmidt, Alexander Haußmann, Applied Optics, 2011