156 POLARIZATION OF LIGHT. 



by Sir David Brewster. In the year 1811 he commenced 

 an extensive series of experiments, with the view of deter- 

 mining the angles of polarization of different media, and of 

 connecting them by a law. The result of these investiga- 

 tions was the simple and remarkable principle, that " the 

 index of refraction of the substance is the tangent of the angle of 

 polarization" Hence, when the refractive index is known, the 

 angle of polarization is inferred, and vice versa ; and we learn 

 from the law that this angle ranges in different substances from 

 45 upwards, increasing always with the refractive power. 



The refractive index being different, for the differently 

 coloured rays which compose solar light, it follows that all 

 the rays of the spectrum are not polarized at the same angle ; 

 so that if a beam of solar light be reflected successively by 

 two glass plates, whose planes of reflexion are at right angles, 

 the reflected beam will never be wholly extinguished, but 

 will be coloured red or blue, according as the angle of in- 

 cidence is the polarizing angle of the more, or of the less re- 

 frangible rays. When the angle of incidence is the angle of 

 polarization corresponding to the most luminous portion of the 

 spectrum, the reflected light is of a purplish tint, formed by 

 the union of the remaining rays in different proportions. 

 These effects are, of course, most observable in highly dis- 

 persive substances. 



(169) The law of Brewster may be presented in another 

 form. We may say that the angle of polarization is such, that 

 the reflected and refracted rays form a right angle. In fact, if 

 this angle be denoted by TT, and the corresponding angle of 

 refraction by p, we have 



sin TT sin TT 



tan TT = iu, or = : 



cos TT sin p 



therefore, cos TT = sin /o, and 



