CIRCULAR POLARIZATION OF LIGHT. Plane-polarized light is altered into circularly polarized light by passing in a par ticular direction through a Fresnel's rhomb. This is a parallelopiped of glass with its faces set at certain angles depending on the ref rac tive power of the glass. The usual experi mental rhomb is a bar of thick plate glass, the ends of which are ground to an angle of 54° and then polished. The light entering one base of the rhomb is twice internally reflected be fore it emerges at the opposite base; and while common unpolarized light passes through the rhomb without suffering alteration, plane-polar ized light has its properties in general complete altered. The final result depends on the in clination of the plane of polarization of the in cident light to the plane of the internal reflec tions. In two cases, namely, when this angle is 0° or 90°, the emerging light is still plane polar ized ;when the angle is 45 the light is circularly polarized; in every other case it is elliptically polarized. In the first case, as will be under stood from consulting the article on POLARIZA TION OF LIGHT, the analyzer, on being applied to test the beam, shows in one position bright light, and on being turned round the principal axis through 90°, total darkness. In the last case — that of elliptic polarization — the ana lyzer shows, on being turned round, a beam of varying intensity, but never complete ex tinction. In the case of circularly polarized light the analyzer, on being turned round, shows a beam of the same intensity in every position of the analyzer, and, in fact, does not at first sight differ from ordinary unpolarized light. When, however, it is examined — not with a Nicol's prism direct, but after a second Fresnel's rhomb has been interposed — it is found to differ very materially from unpolar ized light. The latter is, as we have remarked, unaffected by the rhomb; the circularly polar ized light emerges from the second rhomb plane polarized. It is thus shown how to pro duce and how to recognize circularly polarized light. We now give a few of its most notable
properties.
The light, as we have said, that emerges from the second Fresnel's rhomb is again plane polarized, but it does not emerge precisely as it entered. For, except in one particular position of the two Fresnel's rhombs, the light that emerges from the second rhomb has its plane of polarization changed; the plane is turned round, in fact, through an angle depending on the positions of the two rhombs with regard to the original plane of polarization; and it may be turned round either in a right-handed direc tion, that is, as the hands move on a clock, or in a left-handed direction, that is, a movement opposite to clockwise. We might arrange a set of pairs of Fresnel's rhombs, it is evident, in such positions that each pair should give the plane of polarization of the ray passing through it a farther twist in the same direction, and we might turn it thus through any angle whatever. Such a power as we have imagined in a set of Fresnel's rhombs is possessed naturally by quartz and by a considerable number of solu tions of organic substances and it is known as the power of rotating the plane of polarization. When a beam of homogeneous light has passed through the polarizer, and the analyzer is placed in the position of total extinction of the ray (see POLARIZATION OF LIGHT), on introduc ing a plate of quartz the light reappears; but on turning the analyzer round, either in a right handed direction or in a left-handed direction (whence the names), extinction is again ob tained. Quartz is named right-handed quartz or left-handed quartz according to the direc tion in which the analyzer must be turned. The difference between right-handed and left-handed quartz is due to the fact that the right-handed circular component travels faster in the for mer and slower in the latter. The amount of the angle through which it must be turned de pends on the thickness of the plate of quartz.