The first clues to the existence of polarized light surfaced around 1669 when Erasmus Bartholin discovered that crystals of the mineral Iceland spar (more commonly referred to as calcite) produce a double image when objects are viewed through the crystals in transmitted light. This interactive tutorial simulates viewing of a ball-point pen and a line of text through a crystal of Iceland spar, producing a double image.

To operate the tutorial, position the mouse cursor over the crystal and drag it across the image containing the ballpoint pen and text line. When light reflected from the text or pen is incident upon the surface of the Iceland spar crystal, it is refracted into two wavefronts, each polarized at right angles to one another, and traveling at different velocities. This splitting of light is known as double refraction or birefringence (derived from the term bi-refraction). Although not illustrated in this tutorial, when the Iceland spar crystal is rotated, one of the images remains stationary, while the other precesses around the first. This is due to the strong birefringence in calcite, which separates the two refracted light rays by a wide margin. One of the light waves, termed the ordinary ray, travels straight through the crystal (its image remains stationary), while the other ray is refracted to a significant degree. The refracted ray is termed the extraordinary ray, and is the one that precesses around the ordinary ray.

The ordinary and extraordinary light rays, when passed through a polarizer, have permitted electric vibration directions that are mutually perpendicular. The extraordinary ray always vibrates in a plane that joins it and the ordinary ray, while the ordinary ray always vibrates at right angles to this plane. The speed with which the light rays pass through the crystal also varies. Calcite has what is termed negative birefringence so the ordinary ray travels slower than the extraordinary ray.

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