Most living organisms have evolved to be responsive to light. Light is made up of alternating electric and magnetic fields that oscillate perpendicularly to the direction of travel. In unpolarized light, these fields oscillate in different planes (perpendicular to the propagation of light), but after interacting with a polarizer these oscillations occur in single plane, producing polarized light.
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Upon scattering in the atmosphere or reflecting off water or waxy leaves sunlight becomes linearly polarized, resulting in a particular pattern with the e-vector parallel to the surface. Light also gets polarized when reflected off moist skin, fur, or body surface of certain invertebrates. Rayleigh scattering also produces polarization in scattered light, with 100% polarization in air and <50% polarization in underwater.
Interestingly, in addition to the intensity and wavelength of light, certain life forms have evolved to respond to the polarization state of light. Polarized light can provide information about the position of the sun, presence of a water body, presence of a predator or food source or the presence of suitable substrates for egg laying among other things. For many life forms on the move, the pattern of linearly polarized skylight is an important navigational cue. Many insect species (such as honey bees, desert ants and dung beetles), fish, amphibians, reptiles and birds can detect polarized light and use it for navigation, sexual signaling or even detecting water.
Although humans have not evolved to depend on polarized light for survival, we have made use of the polarized light for navigation purposes using crystals (their ability to detect polarized light is very limited to some degree, though not impossible). The dominant seafarers of the North Atlantic, the Vikings, navigated skillfully for long distances across open sea with the help of polarized sunlight, just like some insects. It is proposed that they used a birefringent crystal, functioning as a linearly polarizing filter, referred to as ‘sunstone’ in one of the Viking's sagas and used to determine the azimuth direction of the Sun. Humans can, under certain conditions, see polarized light in the shape of a Haidinger’s brush (a yellowish form of light); however, its function in the life of humans is not yet known.
Unlike humans, insects have photoreceptors that are intrinsically sensitive to polarized light. Linearly polarized sunlight is used by insects such as the desert locust, monarch butterfly, cricket and fruit fly to help orient their behavior based on the information passed on through their complex optic system, and detailed neural responses which are integrated with unpolarized light responses. Drosophila melanogastor, the cinderella of genetics, is a promising candidate for elucidating ‘both the cellular basis of navigation and mechanisms of directed dispersal (of the insects) on a landscape scale.’
Only in the recent past has the understanding of vertebrate polarization increased significantly. ‘Mechanisms of polarization sensitivity, neural processing of polarization information and behavioural evidence of polarization vision and associated visual ecology’ are the key areas of research in this field.
Certain bird species, such as migratory birds, take advantage of the polarization pattern produced by sunlight ‘to calibrate their orientation systems.’ The degree of sunlight polarization is highest when the sun is near the horizon at twilight, and all the compass cues are within the visible range; this is when the birds initiate migration. Researchers are beginning to understand the physiological mechanisms of avian polarization vision. Similar to birds, the greater mouse-eared bat (a flying nocturnal mammal) also uses the polarized light of the evening sky to calibrate their inner compass.
A complex system of receptors, compass neurons and behavioral responses have helped various life forms detect linearly polarized light and employ it for survival. The ability of visual systems to extract, distinguish, and quantify these properties of electromagnetic waves determine the behavioral, anatomical and physiological evolution of life.
References
- Did Vikings travel by polarized light?
- Polarisation vision
- Bats use the evening sky’s polarization pattern for orientation
- Sensing Polarized Light in Insects
- Perceiving polarization with the naked eye: characterization of human polarization sensitivity
- Polarized Light and Polarization Vision in Animal Sciences
- Celestial navigation in Drosophila
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