Page:The New International Encyclopædia 1st ed. v. 12.djvu/276

LIGHT. 250 LIGHT. bctweon these two positions the emerging light lia.s its inaxiiuum inlcnsily. This observation can be accounted for only if it is assumed that, alter the waves have emerged from the first pUite, the vibrations in the cther arc all in parallel straight lines perpendicular to the direc- tion of the rays, the direction of the vibration being in some way connected with the properties of the crystal of tourmaline; this means that the crystal transmits vibrations in a definite direction only, which is fixed by its crystalline properties. Similarly, the second crystal will transmit only those vibrations which arc in straight lines parallel to some line fixed by its crystalline prop- erties. If, therefore, the waves incident upon the second plate have their vibrations iiarallel to this fixed direction — and this will happen twice in one complete revolution of this plate around an axis parallel to the ras — the waves will be entirely transmitted; whereas, if the direction of the vibration in the incident waves is at right angles to the fi.xed direction in the second plate, no light c.in be transmitted; and for positions where these two important lines make an acute angle with each other, a component of the energy of the waves passes through. The waves after passing the first plate of tourmaline are said to be 'plane polarized,' meaning that all the vibra- tions in the waves are now in parallel straight lines. . test for j)lane polarized waves, further, is to see if they can pass through a thin plate of tourmaline with maximum intensity for two positions of the plate, and he cut o(T completely for two positions at right angles to the other two. In this way it may be shown that each of the beams of light transinitled 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 onlinarj' light falls upon a' plane face of any isotropic transparent substance, e.g. glass or water, both the reflected 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 cases: and for a definite angle of incidence for each such sub- stance all the reflected light will be plane polarized. (DilTerent kinds of plane polarized light arc distinguished by saying that they are 'polarized in different planes.' ]5y definition the 'plane of polarization' of light polarized bv 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 Rreirstrr'x law.) TTie refracted light in this case is a mixture of ordinary light and plane polarized light. The two methods most in use for producing plane polarized light are to allow ordinary light to be reflected from glass at the polarizing angle, and to allow ordinary light to pass through a 'Kicol's prism.' This consists of two pieces of a crystal of Iceland spar cut in a definite man- ner and cemented together by Canada balsam; Iceland spar is a uniaxial cry.stal and so the incident light is broken up into two trains of waves, the ordinary and the extraordinary; when these reach the layer of Canada balsam, the former suffers total rellection. while the latter is transmitted; thus the emerging light is plane IHilarized. it is found, too, that the light scai- tercd by fine particles, e.g. the blue light of tin- sky, is plane polarized. It js passible by several methods so to altei plane polarized light that all the vibrations in the ether as the waves pass are in circles or in ellipses; such light is called 'circularly' . 'elliptically' polarized. Ordinary light such ; comes from the sun or a lamp can be considen-i as a mixture of all kinds of polarized liglii. plane, circular, and elliptical. (If plane polu ized light is rellected from a metallic surfii. it becomes elliptically polarized.) If ordinarj- light is incident upon a thin plaii of an}' doubly refracting substance, it may !«■ shown that two beams of plane polarized light — with their vibrations at right angles to each other — will be transmitted; and further. the-.c beams will travel through the plate at dill'crent speeds, so that on emerging one will be slightly in advance of the other so far as the phase of the waves is concerned. If these two beams now enter a Xicol's prism, only a component of en^ ' will be transmitted, for the prism allows to pa vibrations in a definite direction only, whirh direction is not in general that of the Vibrati"ii in either of the two beams. If the light which falls upon the thin plate is itself plane polarized, it will be broken up as before into two beam-. and they will be recombined into a plane polar ized beam by the Xicol's prism. The two com- ponent beams arc in different phases ; and. if one is retarded by an amount equal to half a wave-length for anj- color or any odd number of half wave-lengths, then, when these beams are combined by the Xicol .so as to have their vibrations all in one direction, the waves of this particular color will be absent, owing to inter ference, if the intensities of the two coincidinL' beams are the same, and if the incident light i- white the emerging light will be colored. (If the incident light had not been plane polarized, there would not have been a definite relation between the intensities of the beams coinciding in the Xicol's prism.) This experiment offers a most delicate test of the double refraction of any substance: place two Xicol's prisms in line, and turn one around its axis until no light is trans- mitted through the two — they are said to be 'crossed:' now introduce between them the sub- stance to be investigated. If it is doubly refract- ing, light will now be transmitted through the Xicol's prisms. This is sometimes called 'de- polarization.' For a full description of the other properties of polarized light, some large treati.sc should be consulted. ETTTKR AXD M.TTER. Among the phenomena of light which can be explained only by assuming some definite con- nect itm between the ether and matter, are dis- persion, both rcgiilar and anomalous: absorption and emission: color: fluorescence and phosphores- cence: rotation of the plane of polarization by quartz and other bodies: metallic reflection; the various magnetic actions on light, etc. Pi.spERSio.x. As has been already explained.