POLARISATION 



POLARITY 



of this relative retardation anil to tlie position of 

 the principal plane of the interposed film, their 

 resultant, that which reaches the analyser, may l>e 

 a beam plane polarised in the original plum 1 , plane 

 jMilarised in another piano, elliptically polarised, or 

 circularly polarised. In all these cases except the 

 liiM, the" analyser lets some light through. If m 

 substitute for the analyser a doubly-refracting 

 crystal, there will in general l>e two images seen on 

 tooting through : hut as lliis crystal itself intro- 

 duces relative retardations, the result of which 

 depends on the wave-lengths i.e. on the colours 

 the different wave-lengths may give different rela- 

 tive intensities in the two images : some wave- 

 lengths may predominate in the one image, the 

 rest in the other ; the two images may thus in- 

 coloured ; and when coloured they will be comple- 

 mentarity coloured. The phenomena of colour 

 produced bv the reaction of polarised light upon 

 various douoly-refracting crystals and films, c. 

 all which colom -phenomena are due to varying 

 relative retardations of ordinary and extraordinary 

 rays in doubly-refracting media, and are either 

 uniform all over the resultant wave-front or vary 

 with respect to particular parts of it are of great 

 variety and extreme beauty. For an account of 

 these we refer to Thomas Preston's Theory of Light 

 (Lond. 1890). 



A beam of plane polarised light may be recognised 

 by means of a crystal of Iceland spar. Paste a piece 

 of paper with a pinbole in it on one end of the 

 crystal ; look through, turning the crystal round ; 

 each of the two images waxes and wanes and dis- 

 api>ears alternately with the other. In partially 

 polarised ordinary light, and in elliptically polarised 

 light, the two images wax and wane alternately 

 with one another, but do not disappear. In 

 circularly polarised ami in ordinary liglit the two 

 images remain equal to one another, and present 

 no variation of intensity. Circularly or elliptically 

 polarised light is converted by a plate of mica of 

 proper thickness into plane polarised light ; natural 

 light, unpolarised or partially unpolarised, is not 

 so affected by the same plate of mica. These 

 criteria enable the character of a given beam of 

 light to be readily recognised. 



The name of Rotatory Polarisation is given to the 

 phenomenon observed when a beam of plane polar- 

 ised light is sent through a slice of quartz cut 

 parallel to the axis. The plane of polarisation is 

 found to have been rotated, and that into a different 

 position for each component colour; so that, witli 

 white liglit incident, a crystal of Iceland spar gives 

 two images complementarity coloured, and varying 

 in colour on rotation of the prism. This property 

 of rotation is shared by many substances even in 

 solution : cane-sugar, grape-sugar, camphor act 

 like quartz, rotating the plane of polarisation to 

 the right (dextro-rotatory); fruit -sugar and starch 

 rotate the plane to the left (laevo rotatory ). U|xm 

 this property are based various instruments for the 

 quantitative estimation of saccharine w>luti m-, 

 called saccharimeters. If the liglit whose plane 

 has been rotated lie reflected back through the 

 plane-rotating medium, the rotation is reversed, 

 and the liglit emerges polarised in the original 

 plane. A somewhat similar phenomenon, though 

 much less pronounced, is observed on passing a 

 beam of light through heavy glass in a strong 

 magnetic field ; but here, if the path of the tight be 

 reversed by reflection, the rotation of the plane is 

 not reversed but doubled. 



As to the direction of vibration in a plane 

 polarised ray, a ray polarised by reflection is -aid 

 to be polarised in the plane of incidence i.e. in a 

 plane containing Initli incident and reflected rays: 

 the question is whether the vibration is in this 

 plane or at right angles to it. Fresnel worked out 



the consequences of the vibration being at right 

 angles to this plane, and arrived (on the assump- 

 tion that the density of the ether ill two media, at 

 whose hounding surface reflection takes place, is 

 different in the two media, while its elasticity 

 is the same in both) at consequences consistent 

 with experiment. Neumann and MacCnllagh, from 

 a contrary hypothesis as to the elasticity and 

 density of the ether, and on the hyjMtthesis that 

 the vibrations are parallel to the plane of polai 

 tion, arrived at optical conclusions which, so far as 

 it is possible to test them by experiment, are 

 equally consistent with observation. Clerk-Max- 

 well's electric or electro-magnetic theory of light, 

 confirmed by Hertz's researches (see MAHNKTISM), 

 requires that there should l>e an iindulatory pro- 

 pagation of electric disturlinces at right angles to- 

 the plane of polarisation, ami of magnetic dis- 

 turbances parallel to that plane. 



Polarisation of light is useful in several ways. 

 A polariser can be made to cut off the glare from 

 the surface of water while we look into its depths ; 

 or to cut off a large portion of the light which is 

 reflected from ha/.e and oliscures our view of land- 

 scape ; or it may be used in examining the light of 

 the sky, which is partly polarised, because due to 

 rclleetion (see SKY). A polariser and analyser are 

 of use in examining the strained condition of glass 

 which, when heated or lient, &c,, or too suddenly 

 cooled, will give rise between crossed prisms to- 

 phenomena analogous to those produced by a 

 doubly-refracting crystal ; and they are also of use 

 in low-power microscopic work for the examination 

 or identification of crystals and of many organic 

 structures. Crossed prisms have also been used to- 

 rediic.e the intensity of a ln-am of light to any 

 required percentage for photometric purposes. 



(2) Polarisation of Dielectric. The condition of 

 the dielectric or medium lietween two opposite 

 charges of electricity : a condition of stress. 



(3) Polarisation of a tlalvantc Cell. Production 

 of a reverse 'electromotive force' by tin- deposi- 

 tion of element 1 * of the electrolyte uiioii, or their 

 combination with, the plates of the cell. 



(4) Poliirtntitinii ';/ Klrrlrnde*. An entirely 

 similar phenomenon in an electrolytic cell. When 

 the battery is taken off, a reverse current flows 

 from the electrolytic cell ; this is the basis of the 

 gas battery and of the modern accumulator (see 



ELECTRICITY ). 



Polarity, in physical science, a word of various 

 application ; hut in all its uses there is present 

 tin- idea of a directed quantity or Vector (q.v.). 

 A sphere, situated in space, is a perfect type of 

 all-sidednesH, presenting the same aspect in every 

 direction. Let this sphere, however, begin to rotate 

 about some diameter, and at once it becomes 

 a polar body ; it becomes possessed of polarity 

 (see I'nl.KS). Looked at fioni one end. it appeals 

 lot.-iting clockwise; looked at from the other, it 

 appears rotating counter-clockwise. A similar 

 polarity is acquired by a liody of any shape when 

 it is set spinning about some axis. Hence we 

 may take lotatiou as a very perfect illustration 

 of kinematic polarity. 



1'ethaps the most familiar example in physics of 

 a polar body is the magnet. Its pohiriu is a t.. 

 polarity, the ends or poles of one magnet having a 

 selective action upon the ends or poles of another. 

 This particular action is, however, only one of a 

 host of manifestations of what is known as Mag- 

 netism (q.v.) ; and the general tendency in modern 

 theory is to explain all magnetic phenomena a* 

 lM-ing essentially rotational. Thus, again, from a 

 physical or dynamic point of view, we conceive of 

 rotation as a true type of polarity. The phenomena 

 of statical electricity have also been discussed as 

 analogous to certain phenomena in vortex motion. 



