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Seeing the Third Quality of Light Polarization Vision
May 1, 2008

Sun is the main source of light for Earth. Without light, there would be no sight for us. Light is nothing but an electromagnetic wave which has three fundamental properties and with naked eyes humans are capable of sensing only two qualities of light – brightness (intensity) and color (frequency). We are essentially blind to the third quality of light. The third property is polarization. The direction in which the electric field oscillates as it propagates is known as polarization. Although unable to naturally sense polarization, we have still been able to measure and analyze polarization in our environment. Polarizing filters are used in photography, certain kinds of sunglasses, digital watches, and laptop screens. Polarization is also used in the entertainment industry to produce and show 3-D movies. We wear polarized sunglasses, for example while fishing, to filter out the glare from polarized light that is reflected off the water's surface. This makes the water more transparent and thus we can more easily see fish swimming in the water. The knowledge we have accumulated over the years leaded us to use polarization in our daily lives and scientifically our understanding of polarization information is still limited. On the other hand, the ability to analyze polarized light is widespread among animals. Here we will explore how animals make use of polarization information available in light.

Solar radiation is unpolarized before entering the earth’s atmosphere. Unpolarized light is a mixture of photons having randomly oriented electric fields. According to the simplest theory (Rayleigh), when unpolarized sunlight scatters from atmospheric constituents (gases, aerosol particles, water droplets, ice crystals), it becomes partially polarized, depending on the scattering angle - the angle between the incoming (direct solar) and outgoing (skylight) rays. Unpolarized light can also undergo polarization by reflection off of nonmetallic surfaces such as asphalt roadways, soils, racks, snow fields and water. Therefore, there is an abundance of polarized light in natural environments in various forms. Recently, it has become apparent that animals can take advantage of these rich sources of information in the underwater world, on the water surface, and in the terrestrial habitat that are of celestial polarization patterns. They utilize this polarized light prevailing in their visual worlds in various ways associated with their behavioral tasks like navigation, communication, mate recognition, eggs laying, detection of water surfaces, enhancement of visual power (similar to colors), or perhaps even camouflage.

The skylight compass

The best understood use of polarization is the skylight compass of insects. The orientation of the electric field changes with the position of the sun. This can make the sun as a compass usable even when the sun is obscured. In 1949, Nobel laureate Karl von Frisch discovered that when the sun is not visible, honey bees can orient their flights and communication dances by means of the extensive patterns of polarized ultraviolet (UV) skylight1. For clear sky, these patterns are quite regular and depend so strongly on the position of the sun. It is amazing to see how these little hard-working creatures come programmed to use them to calculate the sun's location.

Since von Frisch’s pioneering work, several other researchers investigated polarization vision and found that the polarization pattern of the sky offers many other insect species (desert ants, dung beetles, field crickets, and house flies) a reference for visual compass orientation2. For example, desert ants were shown to make long and tortuous foraging walks, but use the sky polarization pattern to return to their nest on a straight line3. They are able to continuously compute their present location from their past trajectory and, as a consequence, to return to the starting point by choosing the direct route rather than retracing its outbound trajectory.Moreover, interestingly enough, researchers discovered that one species of dung beetles navigate by using million-time dimmer polarization patterns of moonlight. Dung beetles use it as an orientation guide to leave their food source in a straight line to avoid aggressive fights4. To find out how the beetles are able to use the polarized light of the moon to navigate, researchers observed the beetles under the night sky. On nights when the moon was visually clear, the beetles continued to forage and roll their dung balls in a straight line. On moonless or cloudy nights the beetles could not maintain a straight path.

Reflections from water

In nature, important reflections come from water where the polarization distinguishes between water and other reflective surfaces. Horizontally polarized UV light reflected from the surface of water is the main optical cue for habitat finding by insects living in, on, or near water. Weak UV light emitted by a horizontal surface below flying backswimmers can cause the animals to turn their flight paths vertically downward, bringing them to the horizontal surface6. Polarization sensitivity has, likewise, been demonstrated in crustaceans, like in the shore-living water flea Daphnia pulex. These animals were shown to swim toward polarized light, which in nature would lead them away from the shore towards deeper water7.

Human activity can have overwhelming effects on the natural environment and man-made objects, such as crude or waste oil surfaces, asphalt roads, glass surfaces, or plastic sheets used in agriculture are unfortunately more attractive to water-seeking polarotactic insects than the water surface itself. This effect can be very dangerous for polarotactic insects as these objects function as insect traps. Researchers have observed that every year, in May and June, swarms of mayflies mate, not above lakes and rivers, but above dry asphalt roads and lay their eggs in vain on dry asphalt roads or car-bodies. The horizontally polarized light from these surfaces mimics a highly polarized water surface.8 .

Ocean animals

For many ocean animals, sensing polarization may be even more important than sensing color. One possible use for polarization in the ocean (and elsewhere) is signaling: communicating with neighbors, rivals, and potential partners. Recent discoveries have shown that stomatopods (Mantis shrimps), a sort of shrimp found on reefs around the world, use special body areas to communicate with polarized light (Fig.6)9. Polarized light can also be used to ‘break the camouflage’ of aquatic organisms because, although from most viewing angles they match the color of the water behind them, the nature of the polarization is quite different. Researchers have found that transparency of aquatic organism to avoid detection can be broken with the help of polarization sensitivity10. In their experiment, they observed that squid detect zooplankton prey under partially linearly polarized lighting 70% greater than those achieved under non-polarized illumination.

In summary, polarization is central to most of the animals’ lives. It is abundant in the nature in various forms. Here, we have given only couple of examples of ways of various animals’ exploitation of polarized-light information. It seems, as researches continue, that the already long list of animals utilizing polarized light will get even longer as we learn more about it. Yet, even these mentioned examples above are enough to help us realize how perfectly these small animals have been created, and how well they are taken care of in their daily lives when they navigate, communicate, recognize a mate, lay eggs, detect water surfaces, or perhaps even break camouflage. Here, it seems necessary to observe that "The tiny body of a fly is connected with most of the elements and causes in the universe; indeed, it is a summary of them. If it is not attributed to the Pre-Eternal and All-Powerful One, it is necessary for those material causes to be themselves present in the immediate vicinity of the fly; rather, for them all to enter into its tiny body; and even for them to enter each of the cells of its eyes, which are minute samples of its body." We refer the interested reader to Said Nursi's reputable article of “A Treatise on Nature"13 and conclude with his aphorism: "He who created the eye of the mosquito is the one who created the sun."