Polarisation. (1) Of Light.—An ordinary narrow beam of sunlight has no sides, and is always divided into two equal beams by a crystal of Iceland spar; but if it has once been reflected from glass or water, it will then be found in general that different results, as regards the intensities of these two beams, are produced by turning the crystal of Iceland spar round the axis of the beam into different successive positions. The beam is no longer the same all round, but has acquired sides. On the vibratory or undulatory theory of Light (q.v.) this shows that the vibrations must be transverse to the direction of propagation (see POLARITY). Suppose a long cord, fixed to a distant wall, to be held in the hand; apply a sharp up-and-down movement; an up-and-down wave will run along the cord to the wall; this illustrates the mode of vibration in a beam of plane polarised light. Make the hand move in a circle, in a direction contrary to that of the hands of a clock; a wave will run along in the form of a screw; this screw will have the form, and will advance after the fashion, of a corkscrew; this illustrates the mode of vibration in a right-handed circularly polarised beam of light. Make the hand move in a circle clockwise; the wave-screw advances in a left-handed fashion; this illustrates left-handed circularly polarised light. Make the hand move in an ellipse; an elliptical disturbance travels, screw-fashion, right- or left-handed as the case may be; this represents elliptically polarised light. Communicate a series of disturbances of the greatest irregularity in which no one direction, up or down, right or left, has on the whole any predominance; the irregular succession of transverse disturbances which will travel along the cord will represent the vibration in a beam of common or natural light. Assume that while communicating these irregular disturbances the hand is hampered but not disabled with reference to any particular direction, say up and down; the vibrations in that direction are on the whole less than those from right to left; and the whole complex of irregular disturbances would, if they wrote their own path, tend to fill up an ellipse with their trace-marking rather than to fill up a circle, as the vibrations in common light would tend to do; this would represent the nature of the vibrations in partially polarised light. Now suppose a slot in a board, which will allow the cord to swing from end to end of the slot, but will not allow the cord to swing athwart the slot; all those oscillations or components of oscillation which are parallel to the slot will be able to traverse the slot; but those which are at right angles to these will not be allowed to pass. On endeavouring to transmit through the slot the complex of oscillations which illustrate the vibrations of common or natural light, it will be found that no motion at right angles to the slot is transmitted, and that what does pass through is a complex of irregular oscillations restricted to the plane of the slot. A second slot, at right angles to the first, will cut off the whole of what passes through the first; and the propagation of transverse oscillations along a cord may thus be entirely checked. If, however, the second slot be parallel to the first, all the oscillations transmitted by the first may pass through it also; and if it lie in an intermediate direction, the second slot will allow a proportion to pass, which depends upon the angle between the two slots, being proportional to , where is that angle. The first slot illustrates the functions of a polariser; the second illustrates those of a second polariser or analyser. A polariser reduces incident common light to a plane polarised condition, and an analyser at right angles to the polariser will quench it altogether.
The phenomena of polarised light were first observed in sunlight reflected from water or glass. Common or natural light so reflected is always, except when it retraces its path by direct reflection, more or less partially 'polarised by reflection.' The polarisation is more or less complete according to the angle of incidence. At one particular angle of incidence the reflected light is as nearly plane polarised as the particular reflecting substance employed can make it. At this angle, the so-called 'angle of complete polarisation,' the reflected and the refracted rays are (or tend to be) at right angles to one another, and , where is the angle between the incident ray and the normal, and is the index of refraction (see REFRACTION). Metal reflectors have no angle of complete, but only of maximum, polarisation; and even among such substances as glass, which are usually said to have an angle of complete polarisation by reflection, it is only those whose index of refraction = 1.46 which can completely polarise common light by a single reflection. In that case the intensity of the reflected plane polarised beam is to that of the original incident beam of common light as 6.52 to 100, or 6.52 per cent. The intensity of light polarised by one reflection is therefore a good deal less than the 50 per cent. which might be secured by any contrivance which effectually acted in a way analogous to the first slot above mentioned. The intensity of light polarised by reflection is greatly improved by using, instead of a single reflecting plate, a pile of plates. A crystal of tourmaline or of iodo-sulphate of quinine will, on the whole, allow only light polarised in one particular plane to pass through; but then it darkens it and colours it. Advantage is accordingly taken of the property of a doubly-refracting transparent crystal, such as Iceland spar, of dividing an incident beam of common light into two equal beams, which are, when they travel in principal sections of the crystal (see REFRACTION, DOUBLE), polarised in planes at right angles to one another, and each of which possesses (absorption apart) half the intensity of the original beam. As these two beams diverge from one another it is comparatively easy to arrange that one of them shall remain parallel to the axis of the incident beam and of the apparatus, while the other is allowed to wander away laterally: and this is the basis of the construction of the prisms of Nicol, Foucault, Wollaston, Rochon, and others, which receive incident ordinary light and transmit plane polarised light.
Two beams of plane polarised light can interfere with one another (see INTERFERENCE) when their vibrations are wholly or partly in the same direction, but not if they be at right angles to one another; and a beam of light polarised in any way can give rise to the phenomena of Diffraction (q.v.).
On interposing in the path of a plane polarised beam of light an analyser, so placed as to allow none of that light to be transmitted, and then placing in the course of the plane polarised beam before it reaches the analyser a thin film of a doubly-refracting substance, such as mica, the field of view may become filled with light. The doubly-refracting film generally breaks the incident plane polarised beam into two plane polarised beams, which are, after emergence from the film, parallel to one another and on the whole coincident if of sufficient breadth. These two beams are differently retarded in the mica; and, according to the amount of this relative retardation and to the position of the principal plane of the interposed film, their resultant, that which reaches the analyser, may be a beam plane polarised in the original plane, plane polarised in another plane, elliptically polarised, or circularly polarised. In all these cases except the first, the analyser lets some light through.—If we substitute for the analyser a doubly-refracting crystal, there will in general be two images seen on looking through; but as this crystal itself introduces relative retardations, the result of which depends on the wave-lengths—i.e. on the colours—the different wave-lengths may give different relative intensities in the two images: some wave-lengths may predominate in the one image, the rest in the other; the two images may thus be coloured; and when coloured they will be complementarily coloured. The phenomena of colour produced by the reaction of polarised light upon various doubly-refracting crystals and films, &c.—all which colour-phenomena are due to varying relative retardations of ordinary and extraordinary rays in doubly-refracting media, and are either uniform all over the resultant wave-front or vary with respect to particular parts of it—are of great variety and extreme beauty. For an account of these we refer to Thomas Preston's Theory of Light (Lond. 1890).
A beam of plane polarised light may be recognised by means of a crystal of Iceland spar. Paste a piece of paper with a pinhole in it on one end of the crystal; look through, turning the crystal round; each of the two images waxes and wanes and disappears alternately with the other. In partially polarised ordinary light, and in elliptically polarised light, the two images wax and wane alternately with one another, but do not disappear. In circularly polarised and in ordinary light the two images remain equal to one another, and present no variation of intensity. Circularly or elliptically polarised light is converted by a plate of mica of proper thickness into plane polarised light; natural light, unpolarised or partially unpolarised, is not so affected by the same plate of mica. These criteria enable the character of a given beam of light to be readily recognised.
The name of Rotatory Polarisation is given to the phenomenon observed when a beam of plane polarised light is sent through a slice of quartz cut parallel to the axis. The plane of polarisation is found to have been rotated, and that into a different position for each component colour; so that, with white light incident, a crystal of Iceland spar gives two images complementarily coloured, and varying in colour on rotation of the prism. This property of rotation is shared by many substances even in solution: cane-sugar, grape-sugar, camphor act like quartz, rotating the plane of polarisation to the right (dextro-rotatory); fruit-sugar and starch rotate the plane to the left (laevo-rotatory). Upon this property are based various instruments for the quantitative estimation of saccharine solutions, called saccharimeters. If the light whose plane has been rotated be reflected back through the plane-rotating medium, the rotation is reversed, and the light emerges polarised in the original plane. A somewhat similar phenomenon, though much less pronounced, is observed on passing a beam of light through heavy glass in a strong magnetic field; but here, if the path of the light be reversed by reflection, the rotation of the plane is not reversed but doubled.
As to the direction of vibration in a plane polarised ray, a ray polarised by reflection is said to be polarised in the plane of incidence—i.e. in a plane containing both incident and reflected rays: the question is whether the vibration is in this plane or at right angles to it. Fresnel worked out the consequences of the vibration being at right angles to this plane, and arrived (on the assumption that the density of the ether in two media, at whose bounding surface reflection takes place, is different in the two media, while its elasticity is the same in both) at consequences consistent with experiment. Neumann and MacCullagh, from a contrary hypothesis as to the elasticity and density of the ether, and on the hypothesis that the vibrations are parallel to the plane of polarisation, arrived at optical conclusions which, so far as it is possible to test them by experiment, are equally consistent with observation. Clerk-Maxwell's electric or electro-magnetic theory of light, confirmed by Hertz's researches (see MAGNETISM), requires that there should be an undulatory propagation of electric disturbances at right angles to the plane of polarisation, and of magnetic disturbances parallel to that plane.
Polarisation of light is useful in several ways. A polariser can be made to cut off the glare from the surface of water while we look into its depths; or to cut off a large portion of the light which is reflected from haze and obscures our view of landscape; or it may be used in examining the light of the sky, which is partly polarised, because due to reflection (see SKY). A polariser and analyser are of use in examining the strained condition of glass which, when heated or bent, &c., or too suddenly cooled, will give rise between crossed prisms to phenomena analogous to those produced by a doubly-refracting crystal; and they are also of use in low-power microscopic work for the examination or identification of crystals and of many organic structures. Crossed prisms have also been used to reduce the intensity of a beam of light to any required percentage for photometric purposes.
(2) Polarisation of Dielectric.—The condition of the dielectric or medium between two opposite charges of electricity: a condition of stress.
(3) Polarisation of a Galvanic Cell.—Production of a reverse 'electromotive force' by the deposition of elements of the electrolyte upon, or their combination with, the plates of the cell.
(4) Polarisation of Electrodes.—An entirely similar phenomenon in an electrolytic cell. When the battery is taken off, a reverse current flows from the electrolytic cell; this is the basis of the gas battery and of the modern accumulator (see ELECTRICITY).