If our observer were further informed that the light he had been observing was the ordinary ray transmitted through a rhomb of Iceland spar, he would be able to determine the azimuth of the principal plane of that Thomb, which would be the same as the principal plane of the rhosub with which he was furnished when the latter rhomb was turned till the extraordinary pencil disappeared.
light poosessing the properties which have just been described as belonging to the ordinary ray In s. rhomb of Iceland spar, however those properties may have been acquired, Is said to be polarized, and the plane to which the properties are related as are those of the ordinary ray to the principal plane of the crystal is called the plane of polariza tion. Tho polarization may be detected, and the azimuth of the plane of polarization determined, by means of a rhomb of Iceland spar, as above described.
The properties of a ray which emerges after extraordinary refraction in a rhomb of Iceland spar, as may be inferred from the description given above, are absolutely identical with those of the ordinary ray, except that they are similarly related to a plane perpendicular to the principal plane. Bence the extraordinary ray is polarized in a plane perpeudietihw to the principal plane, the ordinary ray being, according to our definition, polarized in the principal plane.
If a piece of Iceland spar, or other refracting crystal, be cut into the form of a wedge or prism, the deviation of the two rays, into which a ray incident upon it is in general divided by donble refraction, will not be the same, and thus the rays will emerge in directions inclined to each other, so that their lateral separation will increase with the distance from the prism. Two such raya are still found to be polarized, and their planes of polarization to be nearly perpendicular to each other, being accurately so when the refracting edge is per pendicular to a plane of optical symmetry, or principal plane, of the crystal, and the refraction takes place in that plane.
For more than a century after the original discovery of Huygens, double refraction was the only phenomenon in which light was known to receive the modification called polarization. But in the year 1303, :ileitis made the important discovery that light receives the same modification by reflection at a certain angle from the surface of glass, water, and transparent substances in general, provided their surfaces be smooth or polished, so as to give a regular reflection. The plane of reflection was found to be the piano of polarization of light thus polarized. The angle of incidence required for complete polarization
by reflection was found to vary with the nature of the substance. At an angle of incidence greater or less than this (but not reaching 0' or 90°) the same modification was imperfectly produced, the light never vanishing on rotating the analysing rhomb, but only passing through a minimum in those positions in which et the polarizing angle it vanished altogether. The simplest conception to form of the nature of light thus partially polarized is to regard it as a mixture of common light and of light polarized in the plane of incidence. The refracted light is found to be only partially polarized even at the angle of com plete polarization of the reflected. light, the plane of polarization being perpendicular to the plane of incidence. It may be regarded as composed of a mixture of common light and of light polarized in the plane last mentioned ; and it has been found that at the same angle of incidence the quantity of light polarized in the plane of incidence contained in the reflected beam is equal to the quantity of light polarized in the perpendicular plane contained in the refracted beam. When common light is reflected from a metal, the reflected light is only partially polarized in the plane of incidence, whatever be the angle of incidence.
Shortly after Malus'a discovery of the polarization of light by reflection, Sir David Brewster commenced an extensive series of expe riments on the polarizing angle of a variety of media, which resulted in the discovery of the beautiful law which determines the angle of polarization, namely, that the tangent of the polarizing angle is equal to the index of refraction, which may be otherwise expressed by saying that the angle requisite for complete polarization is that for which the directions of the reflected and refracted rays are perpendicular to each other. This Lew embraces, as a particular case, a law which Mains had already found, that when light is incident on a plate of glass or other medium bounded by parallel surfaces at the angle required for completepolarization of the light reflected at the surface, the refracted light is also incident on the second surface at the angle of polarization for internal reflection. Accordingly a plate of glass, or a pile of plates, may be used instead of a single surface to furnish polarized light by reflection at the proper angle, and a pile of plates has the advantage of giving light of much greater intensity than would be got from a single surface.