POLARIZATION OF LIGHT, an ef fect upon the mode of vibration of light pro duced by reflection, ordinary refraction or double refraction. The term polarisation is due to Newton who in the development of his emis sion theory of light made use of a supposed analogy between the phenomena of magnetism and those observed in optics. The distinction between polarized and unpolarized or as it is sometimes called natural light may be most readily illustrated by means of the phenomena observed when a beam of light is reflected from the surface of a plate of glass or from any other transparent material. At whatever angle a beam may strike the surface a portion will be reflected and the remainder will penetrate the glass following a path the direction of which is determined by the well-known law of ordinary refraction. According to this law sin i/sin r= n, where i (Fig. 1) is the angle of incidence, r the angle of refraction and n the index of the glass or other material. The reflected ray, although weaker than the incident ray, appears, if we judge it by many of its or dinary properties, to be precisely similar to the latter. It is not, however, in all respects the same, for if it be again reflected from the sur face of glass the reflecting power of this second mirror will be found to depend upon its position. If for example both mirrors be so mounted (Fig. 2) that the angle of incidence for each is about 57 degrees and if the second mirror be turned upon an axis so situated as to permit of its rotation without changing the angle of inci dence, it will be found that the second mirror reflects the beam received from the first mirror just as it would any ordinary ray so long as its position is such as not to divert the beam from the plane in which it is first reflected. When, however, in consequence of the rota tion, the plane of reflection at the second mirror deviates from that at the first mirror, the proportion of light reflected diminishes. When the angle between these two planes becomes 90 degrees no light is re flected from the surface of the second mirror. This striking phenomenon was first noticed by Mains in 1811.
It is clear that the beam of light, as the re sult of the first reflection, has undergone some modification which renders it incapable of being reflected the second time under the conditions just stated. Light thus modified by reflection is
said to be polarized.
In accordance with the accepted theory, light consists of exceedingly rapid transverse vibra tions of the luminous ether. These transverse vibrations, the progressive motion resulting from which constitutes the light-wave, are sup posed to be rectilinear. When, however, in the case of a beam of unpolarized light, we attempt to determine the plane in which the vibrations take place we find that they appear to occur equally in all Wanes at right angles to the axis of the wave. This is, indeed, the quality which is expressed by the term unpolarized light. This is not strange, however, when we consider that in one second of time about 500 million million vibrations of the ether have taken place. It is supposed that the plane of vibration gradually shifts so that, in the relatively very long inter val of time necessary to produce an effect upon the eye, vibrations will have taken place in every possible plane to an equal extent.
That this shifting of the plane of vibration of light is really very slow appears from the in genious experiments of Lummer and Gehrcke (Deutsche Physikalische Gesellschaft, 1902), who have shown that there is no appreciable shifting of the plane of vibration during an interval of time sufficient to allow of at least 2,600,000 vibrations of the ether. Better to grasp these relations, imagine a vibratory movement with a period of one second. More than 2,000, 000 years would then elapse before the number of vibrations would equal that executed in the vibration which causes light in a single second of time. In the case of this slow vibration we should expect to be able to detect a slight change in the plane in which the vibrations take place after about four days. Even with this very gradual shifting of the plane of vibration, how ever, it is obvious that in the course of 2,000,000 years the opportunities for variation are so great that one would not any longer be able to determine what the original plane of vibration had been.