Determination of the Plane of Polarisation

The most important experiments to be made with polarised light consist in determining the position of the plane of polarisation, or in measuring the angle through which that plane has been turned by the passage of the light through a column of active substance, such as a solution of sugar, turpentine, or various essential oils, or a piece of quartz.

The simplest method of making this measurement is by the use of a Nicol's or other polarising prism. This is mounted in a cylindrical tube which is capable of rotation about its own axis. A graduated circle is fixed with its centre in the axis of the tube, and its plane at right angles to the axis, and a vernier is attached to the tube and rotates with it, so that the position, with reference to the circle, of a fiducial mark on the tube can be found. In some cases the vernier is fixed and the circle turns with the Nicol. If we require to find the position of the plane of polarisation of the incident light, we must, of course, know the position of the principal plane of the Nicol relatively to the circle. If we only wish to measure a rotation a knowledge of the position of this plane is unnecessary, for the angle turned through by the Nicol is, if our adjustments be right, the angle turned through by the plane of polarisation.

For accurate work two adjustments are necessary:

(1) All the rays which pass through the Nicol should be parallel.
(2) The axis of rotation of the Nicol should be parallel to the incident light.

To secure the first, the source of light should be small; in many cases a brightly illuminated slit is the best. It should be placed at the principal focus of a convex lens; the beam emerging from the lens will then consist of parallel rays.

To make the second adjustment we may generally consider the plane ends of the tube which holds the Nicol as perpendicular to the axis of rotation. Place a plate of glass against one of these ends and secure it in this position with soft wax or cement. The incident beam falling on this plate is reflected by it. Place the plate so that this beam after reflexion retraces its path. This is not a difficult matter; if, however, special accuracy is required, cover the lens from which the rays emerge with a piece of paper with a small hole in it, placing the hole as nearly as may be over the centre of the lens. The light coming through the hole is reflected by the plate, and a spot of light is seen on the paper. Turn the Nicol about until this spot coincides with the hole; then the incident light is evidently normal to the plate - that is, it is parallel to the axis of rotation of the Nicol.

If still greater accuracy be required, the plate of glass may be dispensed with, and a reflexion obtained from the front face of the Nicol. This, of course, is not usually normal to the ^xis, and hence the reflected spot will never coincide with the hole, but as the Nicol is turned, it will describe a curve on the screen through which the hole is pierced. If the axis of rotation have its proper position and be parallel to the direction of the incident light, this curve will be a circle with the hole as centre. The Nicol then must be adjusted until the locus of the spot is a circle with the hole as centre.

When these adjustments are completed, if the incident light be plane-polarised, and the Nicol turned until there is no emergent beam, the plane of polarisation is parallel to the principal plane of the Nicol; and if the plane of polarisation be rotated and the Nicol turned again till the emergent beam is quenched, the angle turned through by the Nicol measures the angle through which the plane of polarisation has been rotated.

But it is difficult to determine with accuracy the position of the Nicol for which the emergent beam is quenched. Even when the sun is used as a source of light, if the Nicol be placed in what appears to be the position of total extinction, it may be turned through a considerable angle without causing the light to reappear. The best results are obtained by using a very bright narrow line of light as the source - the filament of an incandescence lamp has been successfully employed by Mr. McConnel - as the Nicol is turned, a shadow will be seen to move across this line from one end to the other, and the darkest portion of the shadow can be brought with considerable accuracy across the centre of the bright line. Still, for many purposes, white light cannot be used, and it is not easy to secure a homogeneous light of sufficient brightness. Two principal methods have been devised to overcome the difficulty; the one depends on the rotational properties of a plate of quartz cut normally to its axis; the other, on the fact that it is comparatively easy to determine when two objects placed side by side are equally illuminated if the illumination be only faint. We proceed to describe the two methods.