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The fourth-order rainbow
On 11 June 2011 I was able to record the first ever image of the fourth-order rainbow in nature while attempting to reproduce Michael Grossmann's image of the third-order rainbow. The fourth-order rainbow results from sunlight suffering four reflections inside raindrops. It occurs on the sunward side of the sky within the bright glare of the zero-order glow. Consequently, and because it is very faint, it cannot be seen with the naked eye. However, it can be made visible in digital camera images by contrast enhancement and by increasing saturation. An alternative name for this type of rainbow is quaternary rainbow. The press also likes to call it "quadruple rainbow".
I have written a publication on this exciting discovery (PDF): M. Theusner, "Photographic observation of a natural fourth-order rainbow," Appl. Opt. 50, F129-F133 (2011).
This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-50-28-F129. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.
The article has been published in the feature issue of Applied Optics on Light and Color in the Open Air along with the article by Grossmann et al. on the disovery of the third-order rainbow and the pioneering paper by Lee and Laven that sparked the hunt for the higher order rainbows. OSA's press release on these ground breaking papers can be found here. Michael Grossmann et al.'s paper can be downloaded from Michael Grossmann's homepage.
If you want to contact me, you can use the email address stated in the article.
The third-order rainbow (left) and fourth-order rainbow (right).