POLARISATION OF LIGHT. 



13 



light angle ; hence A B C is likewise a 

 right angle, and the angles A B E, G B E, 

 each half a right angle, or45. The effect 

 of the refracting force consequently has 

 been merely to bend the ray of light R G, 

 so as to make it suffer reflexion at the 

 particular angle of 45. 



CHAPTER III. 



Polarisation of Light by ordinary Re- 

 fraction Experiments of Malus of 

 Dr. Brewster Law of the Polarisa- 

 tion of Light transmitted through a 

 Number of Plates Condition of the 

 Light transmitted through one or 

 more Plates. 



Hitherto we have paid no attention to 

 the state of the ray AC, fig. 13, refracted 

 by the transparent surface after the part 

 separated from it by reflexion had been 

 polarised. It might naturally have 

 been expected that it had suffered 

 some change in its properties ; but it 

 was not till 1811 that it was disco- 

 vered that it contained a portion of 

 polarised light*. 



This property will be better understood 

 if we make use of a bundle of glass plates, 

 AB, C D,/^. 17, placed parallel to one 



!>. 17. 



another. Let a ray R A, therefore, fall 

 upon the first plate, A B, of this bundle, 

 at the polarising angle, so that the re- 

 flected ray A B r will be polarised. The 

 transmitted ray E F, emerging at E, 

 will be found to be completely polarised ; 

 but if we receive it upon a plate of glass 

 M N at the polarising angle of 56 45', 



* This discovery was made by Malus. M. Biot 

 made the same discovery about the same time; and 

 in IS 13 Dr. Brewster discovered the same fact by a 

 different ir.ethod, and his Paper on the subject was 

 read to the Royal Society of London before he knew 

 that Malus had anticioated him in the discovery. 



we shall find that it will refuse to be 

 reflected ; whereas A B' does not refuse 

 to be reflected, unless the plate M N is 

 turned round 90 into a plane at right 

 angles to the plane of refraction B A E. 

 Hence w r e conclude 



That when a ray of light is incident 

 at the polarising angle upon any trans- 

 parent body, the whole of the reflected 

 ray is polarised; and a nearly equal 

 portion of the transmitted ray is polar- 

 ised in a plane at right angles to the 

 polarisation of the reflected ray*. 



If we now take two bundles of glass 

 plates AB, C T>,fig. 18, and place them 

 in a similar position, so that the planes 



Fig. 18. 



of refraction in each are parallel to one 

 another, then a ray of light R S, incident 

 at the polarising angle, and polarised at 

 sT by the first bundle, will penetrate 

 the second bundle as at T P ; and not a 

 single ray of it will be reflected by the 

 plates of the second bundle C D. If we 

 now turn C D round its axis, the trans- 

 mitted light PV will gradually diminish, 

 and more and more light will be reflected 

 by the plates of the bundle, till, after a 

 rotation of 90, the ray P V will disappear, 

 and all the light will be reflected. By 

 continuing the rotation of C D, the pencil 

 P V will again appear, and be a maximum 

 at 180, a minimum at 270, and again a 

 maximum at 0, when it has returned to 

 its first position. 



In order to determine the law of the 

 phenomenon, Dr. Brewster provided him- 

 self with 47 plates of crown glass, and, 

 having formed them in succession into 

 bundles of 47, 44, 41, &c. plates, he 

 measured the angles at which the trans- 

 mitted ray was wholly polarised, and 

 obtained the following results : 



* According to an experiment by M. Arago, the 

 one portion is exactly equal to the other; but though 

 this is probable, we think the experiment requires to 

 be repeated under a better form. 



