Double Vision

Perspectives from The Artist's Road

The unusual characteristics of Icelandic spar, a simple calcite mineral, have changed history, influencing everything from navigation, to war, to today’s lasers and fiber optics.

What characteristics made this calcium carbonate so famous? It contains virtually no impurities, unlike other calcites, making it is transparent to both visible and ultraviolet light. Because of these rare properties, Icelandic spar is a natural polarizing filter. Unpolarized light waves from sunlight are moving in all planes simultaneously. Somewhat like a picket fence, light moving through a polarizer gets aligned into one plane only. Curiously, when sunlight enters Icelandic spar, the light is split into two perpendicularly polarized beams which then take two different angles to exit the crystal. This creates a double image of the object seen through the crystal. This effect is called birefringence and is the reason Icelandic spar became so important to our understanding and use of light today.

The polar…