Seeing circular polarization

By Marcus Wilson 22/11/2012

Physicsworld magazine is doing a ‘special feature’ this month on animal superheroes – those with rather unusual physical abilities.

The best of the lot (in my subjective opinion) is the featured-on-the-cover mantis shrimp. Not because of its ‘dactyl clubs’ that can produce a force of 700 N, but because of its eyesight.

The mantis shrimp can see circularly polarized light – something that no other animal is known to do. Polarization describes how the electric and magnetic fields in the light wave are oriented. For example, a horizontally-travelling light wave (say in the x- direction) might have its electric field pointing in the z-direction (vertically) and the magnetic field in the negative y direction. In an electromagnetic wave, the electric field, magnetic field and direction of travel are all mutually perpendicular. We could call that a vertical, plane polarization.

In circular polarization, the electric field moves in a corkscrew-like shape as the wave travels. The corkscrew can spiral one of two ways – hence there are two distinct polarizations which we call left-handed and right-handed. The mantis shrimp can distinguish between the two. It does this by using its own version of a quarter-wave plate – made of a birefringent material – one that has a different refractive index in different directions. That converts a circular polarization to a linear polarization, which it detects via more conventional methods. (There are several animals that can ‘see’ linear polarization – bees are a famous example. There are plenty that don’t distinguish one  polarization from another at all, such as humans.)

The mysterious question is why? Bees use linear polarization to assist navigation (light from the sky is linearly polarized), but what use is distinguishing left-handed and right-handed circular polarizations to a shrimp? There’s a cool research question for someone’s PhD thesis.


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