Circular polarization in nature

Only a few mechanisms in nature are known to systematically produce circularly polarized light. In 1911, Albert Abraham Michelson discovered that light reflected from the golden scarab beetle "Plusiotis resplendens" is preferentially left-handed. Since then, circular polarization has been measured in several other scarab beetles, as well as some crustaceans. In these cases, the underlying mechanism is the molecular-level helicity of the chitinous cuticle.cite journal |title=Imaging polarimetry of the circularly polarizing cuticle of scarab beetles (Coleoptera: Rutelidae, Cetoniidae) |author=Hegedüs, Ramón; Győző Szélb; and Gábor Horváth |doi=10.1016/j.visres.2006.02.007 |journal=Vision Research |volume=46 |issue=17 |month=September |year=2006 |pages=2786-2797 |url=http://arago.elte.hu/PUBLICATIONS/CIKKEK/ANGOL/ScarabCircPol_VR-paper-with-bw-figs.pdf]

The bioluminescence of firefly larvae is also circularly polarized, as reported in 1980 for the species "Photuris lucicrescens" and "Photuris versicolor". For fireflies, it is more difficult to find a microscopic explanation for the polarization, because the left and right lanterns of the larvae were found to emit polarized light of opposite senses. The authors suggest that the light begins with a linear polarization due to inhomogeneties inside aligned photocytes, and it picks up circular polarization while passing through linearly birefringent tissue. [cite journal |title=Circular polarization observed in bioluminescence |author=Wynberg, Hans; Meijer, E.W.; Hummelen, J.C.; Dekkers, H.P.J.M.; Schippers, P.H.; Carlson, A.D |url=http://keur.eldoc.ub.rug.nl/FILES/wetenschappers/10/29/29.pdf |journal=Nature |volume=286 |issue=5773 |pages=641-642 |month=August |day=7 |year=1980 |doi=10.1038/286641a0]

Water-air interfaces provide another source of circular polarization. Sunlight that gets scattered back up towards the surface is linearly polarized. If this light is then totally internally reflected back down, its vertical component undergoes a phase shift. To an underwater observer looking up, the faint light outside Snell's window therefore is (partially) circularly polarized. [cite book |title=Polarized Light in Animal Vision: Polarization Patterns in Nature |author=Horváth, Gábor and Dezsö Varjú |year=2003 |publisher=Springer |isbn=3540404570 |pages=100-103]

Weaker sources of circular polarization in nature include multiple scattering by linear polarizers, as in the circular polarization of starlight, and selective absorption by circularly dichroic media.

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