Iceland spar

The Icelandic sagas of the 10th century record the details of Viking voyages. They describe a mysterious "sunstone", which Scandinavian seafarers used to locate the Sun in the sky and navigate on cloudy days.

Iceland spar, formerly known as Iceland crystal is a transparent variety of calcite, or crystallized calcium carbonate, originally brought from Iceland, and used in demonstrating the polarization of light.

It occurs in large readily cleavable crystals, easily divisible into rhombs, and is remarkable for its birefringence. This means that the index of refraction of the crystal is different for light of different polarization. A ray of unpolarized light passing through the crystal divides into two rays of perpendicular polarization directed at different angles, called double refraction. So objects seen through the crystal appear doubled.

How did that help the Vikings?

Researchers studied a piece of Iceland spar discovered aboard an Elizabethan ship that sunk in 1592. They found that moving the stone in and out of a person's field of vision causes them to see a distinctive double dot pattern that lines up with the direction of the hidden Sun.

Screenshot from the TV show Viking
The polarization of sunlight in the Arctic can be detected, and the direction of the sun identified to within a few degrees in both cloudy and twilight conditions using the sunstone and the naked eye. The process involves moving the stone across the visual field to reveal a yellow entoptic pattern on the fovea of the eye, probably Haidinger's brush.

When light passes through calcite crystals, it is split into two rays. The asymmetry in the crystal's structure causes the paths of these two beams to be bent by different amounts, resulting in a double image.

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