Spark image

Double refraction

Double refraction was observed by Bartholinus in 1669. He placed a crystal of Iceland spar (calcite) above some words written on a piece of paper and found that he obtained two sets of images.

This is explained in the following diagram (Figure 1). When unpolarised light hits a crystal of calcite two refracted rays are formed, one is called the ordinary ray (0), because it obeys the normal laws of refraction, and the other the extraordinary ray (E), because it doesn't!

It was soon discovered that these two rays were totally plane-polarised at right angles to each other. If we could separate the rays then we would have a very good method of producing polarised light.

A method was found by the Scottish physicist William Nicol, who in 1828 devised the Nicol prism. This is simply a crystal of calcite cut through the centre and then stuck together again with a glue called Canada balsam.

The paths of the ordinary and extraordinary rays through such an arrangement are shown in Figure 2. The importance of the glue is that its refractive index is less than that of the calcite for the ordinary ray and greater than that of the calcite for the extraordinary ray. This means that total internal reflection can, and does, occur for the ordinary ray but not for the extraordinary ray.

The polarised nature of the two rays is easy to show using a piece of calcite and a sheet of Polaroid. Place the calcite over a line drawn on a piece of paper and view the double image produced through the Polaroid sheet. Rotate the Polaroid; you will see that each image disappears at certain points in each rotation.

© Keith Gibbs 2011