POLARIZATION, the act of polariz ing or of giving polarity to, the state of being polarized. In galvanism, the pro duction of a secondary current in a gal vanic battery contrary to the principal one, owing to the gradual chemical change in the elements of the battery. This change weakens or may even de stroy, the original current. Many forms of battery recover by rest; in others in genious means are devised to avoid polar ization, and such are called constant batteries.
Polarization of Light.—In optics, a state into which the ethereal undulations which cause the sensation of light are brought under certain conditions. The most familiar and simple form is that of plane polarization. This may be pro duced in various ways, the piece of ap paratus producing such modifications, being called a polarizer. When pro duced, however, the effects can only be perceived by examining them through another piece of apparatus which used alone, would polarize the light, but when used to examine light already polarized, is called the analyzer. The two in com bination, with the necessary adjustments, form a polariscope, of which there are many forms.
Plane Polarization.—When a ray of common light passes through a crystal (not of the cubic system), the atoms be ing so arranged that the elasticity (or other properties affecting motions of the ether within the crystal) are different in different directions, the ether motions are at once resolved into that of the greatest and the least elasticity at right angles to the path of the ray, so dividing the ray of common light into two "plane polarized" rays, polarized in planes at right angles to each other. One of these rays being easily eliminated by total re flection in the Nicol prism, two such prisms form a convenient polariscope. The ray, after passing through the first prism, appears just like common light, only of half the original brilliancy; but on looking at it through the second Nicol, on turning the latter round, we find two positions in which the light from the first Nicol gets through the second unaltered; and two positions at right angles to the former in which it is absolutely stopped, and the second prism, though clear as glass, is absolutely opaque to it. The beam of light appears thus to have acquired sides, and to behave differently according to the relation these sides bear to the position of the prism. Such is the fundamental nature and phenomenon of polarized light. Light is also polarized by reflection from polished transparent surfaces, when incident at such an angle that the reflected and refracted rays make a right angle. In glass, this angle is about 56°.
Chromatic the per pendicular vibrations from a Nicol prism encounter in their path a crystalline film of selenite or mica, whose planes of greatest and least elasticity are arranged diagonally. The perpendicular vibra tions are again "resolved" into two sets, one of which is retarded behind the other owing to the difference in the two elas ticities. The analyzer "resolves" each of
these again, bringing half of each set back into one plane. The two sets of waves are then in a position to exercise interference, and the consequence is that, if the plate or film is of suitable thick ness, the most gorgeous colors are pre sented. It is the same with every sub stance having different elasticities in different directions, and as all "struc ture" presents such differences, polarized light becomes the most powerful weapon of the biologist, revealing structure where ordinary light will not do so.
Circular two rec tangular, equal impulses are given to a pendulum, or to a stretched cord, one of them a quarter-vibration later than the other, the two are compounded into a single circular orbit. Therefore, if a beam of plane-polarized light passes, as in the last paragraph, through a film of mica of such a thickness that one of the two diagonally vibrating sets of waves shall be retarded, while in the film one quarter-vibration behind the other, the two are compounded on emergence into one beam of circularly polarized light. At approximate thicknesses, the light is elliptically polarized. Circularly polarized light is never stopped by the analyzer, but differs from common light in producing polarized effects. The most important of these are: Rotatory the for mer experiment by passing the light from the polarizing Nicol, with its vibra tions in'a vertical plane, through a plate of selenite or mica which gives fine color; the vibrations are then in the two diag onals. Let this light now traverse a "quarter-wave" mica film, with its polar izing planes perpendicular and horizon tal. Each set of rays from the first plate becomes circularly polarized, but the two are in opposite directions, the circu lar movements thus opposing each other. Whenever two circular notions thus meet, as in two circular pendulums clash ing, the tangential motion is destroyed, and the pendulums would both fall back together through the center of the former orbit. It is so in this case; but as one set of rays has been retarded in the plates more than the other, the swing of the ether atoms is no longer in the original plane of vibration. If white light be employed, the many various wave lengths will obviously meet at dif ferent points, and hence rotation of the analyzer will give in succession more or less of the colors of the spectrum. If the quarter-wave film is cut in half, and its position reversed in one-half, the transition of colors will occur in opposite orders in the two halves. Rotary polar ization is of the greatest practical im portance.
Polarization of polarizing of rays of heat by reflection and by re fraction.
Polarization of the name given by Faraday to the produc tion of alternate layers of positive and negative electricity in the medium sepa rating an electrified and an unelectrified body.