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The sensory world of polarization

Insects and some other animals have evolved eyes that can detect linearly polarized light. The light reach the photoreceptors directly from the sun or after being reflected against surfaces like water bodies, at which horizontally linearly polarized light is formed. Some animals seem to use the skylight polarization pattern for orientation, while other animals like most humans, cannot see polarized light. The pattern of skylight polarization form a symmetry plane in relation to the sun, and will change as the sun moves across the sky. Under overcast skies it has been claimed to be difficult or even impossible to see this pattern, and to use it to pinpoint the sun for navigation.

A man dressed in orange is standing in the snow with some measurement instruments.
Gabor Horváth measuring the pattern of skylight polarization during an expedition (Beringia 2005) to the North Pole. Photo: Susanne Åkesson

How to navigate without the sun

In the Old Icelandic Saga by King Olav, it was told that a “Sunstone” could be used by the Viking King to tell the direction to the sun under completely overcast skies, when the eyes could not see any trace of the sunlight. In our work we have investigated this possibility by measuring the degree and direction of the pattern of skylight polarization under different cloud conditions and in fog, confirming the possibility and discussing the old navigation tools used by Vikings during their long sea-crossings. 

Why the zebra got its stripes

I work in close collaboration with optical physicist Dr. Gabor Horváth and his research team at Eötvös University, Budapest. We study Viking navigation, but also host detection in horseflies and optical host protection in mammalian hosts like, horses, cows and zebras. Some of our work has received substantial media attention. We received the IgNobel Prize in Physics in 2016 for a study on why white horses are not attracting as many horseflies as black horses. We have also investigated why zebras have evolved a striped coat pattern, and propose it has to do with an evolutionary arms-race where zebras have evolved striped coats as optical protection against attacking bloodsucking Tabanide flies, that may irritate grazing mammals and spread lethal diseases. 

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