POLARIZATION. All the phenomena so far dis missed depend for their explanation upon the assumption that light is due to wave-loot ion : and therefore similar ones may be observed with all kinds of wave-motion; for instance• aerial waves may be reflected. refracted. and diffratit•d, and may be made to interfere. But there are certain other phenomena in the case of light whirl establish the fact that the ether-waves which produce are totast-,rsc. that is. the vibra , tidal in the ether is at right to the direc tion of advance of 02 waves. In other vibration in the ether is in the wave front as the waves advance in the pure ether or through i•otropie material bodies. This may be proved in several ways. Perhaps the simplest is this: cut two identical plates of a crystal of tourmaline: let ordinary V.11111' Iltdlt frail the sun or from a lamp through are I f these plates and then let it fall perpendicularly upon the other; if this second plate is turned slowly around an axis parallel to the of light. it will be noticed that. one complete revo lution of the plate through 3t10'. there are two differing by PAO for which no light emerges from the second plate, while half way between these two positions the emerging light has its maximum intensity. This observation can he accounted for only it it is assumed that, atter the wares have emerged from the first plate, the vibration, in the ether are all in parallel straight lines perpendicular to the direc tion of the rays, the direction of the vibration fling in some way connected with the properties of the crystal of tourmaline; this means that the erystal transmits vibration, in a definite direction only, which is fixed by its crystalline properties.
Similarly, the second crystal will transmit only those vibrations which a Fe in straight lines parallel to some line fixed by its crystalline prop erties. lf, therefore, the waves incident upon the second 'date have their vibrations parallel to this fixed direction—and this will happen twice in one complete revolution of this 'date around au axis parallel to the rays—the wares will lie entirely transmitted; whereas, if the direction of the vibration in the incident waves is at right angle, to the fixed direction in the second plate, no light e•n be transmitted; and for positions where these two important lines all acute angle with other, a component of the energy of the waves passe- through. The waves after
passing the first plate of tourmaline arc said to be 'plane polarized,' meaning that all the vibra tions in the wave, are now in parallel straight lines. A test for plane polarized waves, further, is to see if they can pa-s through a thin plate of tourmaline with maximum intensity for two positions of the plate. and be cut off completely for two positions at right angles to the other two.
In this way it may be shown that each of the beams light transmitted by a doubly refracting substance is plane polarized, but that the position of the tourmaline plate required to produce ex tinction of one beam is perpendicular to that required to extinguish the other: the two beams are said to be 'polarized in planes at right angles' to each other. (In fact tourmaline is itself a uniaxial double refracting substance: but unless it is made in a very thin plate, only one kind of ray is transmitted, the `extraordinary'—the `ordinary' rays are absorbed in the crystal.) Similarly, if ordinary light falls upon a plane face of any isotropic transparent substance. e.g.. ;lass or water, both the refleeted and the trans mitted waves are found to contain some that are plane polarized—but in planes that are at right angles to each other for the two eases: and for a definite angle of incidence for each such soh. stance all the reflect ed light will be plane polarized. (Different kinds of plane polarized light are distinguished by saying that they are in different planes.' By definition he 'plane of polarization' of light polarized by reflection is taken as the plane which includes the incident and reflected rays.) This angle of incidence is called the `polarizing angle' for the transparent substance. (The tan gent of the polarizing angle equals the index of refraction. This is known as Br•wster's In The refracted light in this ease is a mixture of ordinary light and plane polarized light.