POLARISATION OF LIGHT. 



11 



Polarisation of Light at the separating 



surfaces of two media. 

 When a ray of light is incident at the sepa- 

 rating surface of bodies of different refrac- 

 tive powers, it is polarised at angles whose 

 tangent is equal to the index of refraction. 

 In this case, if m is the index for the 

 most refracting body, and m' that of the 

 least, such as glass and water, then the 

 index for the separating surface will be 



or - - = 1.1415, which is the tan- 



m' 1.336 



gent of 48 47'. This case is shown in 



Fig. 15. 



fig. 15, where P Q is the separating sur- 

 face of the water M N, and the glass 

 beneath, R A the ray incident on the 

 separating surface, A M the ray reflected 

 at the separating surface, and A C the ray 

 refracted by it. " In this case the ray A C 

 ma}' be shown to be at right angles to 

 A M by the same reasoning already used. 

 From this law a curious consequence 



is deducible, which Dr. Brewster verified 

 by experiment. If the water is laid in a 

 parallel stratum upon the surface of the 

 glass, there is no angle of incidence upon 

 its first surface at which it can fall that 

 will arive an angle of incidence upon the 

 separating surface P Q, capable of po- 

 larising the pencil. In short, the polar- 

 ising angle would, by the law, be great- 

 er than 90. The polarisation of the 

 incident pencil increases from 0of in- 

 cidence up to 90, where it is nearly com- 

 plete. 



Hence we see the reason of giving the 

 water mfig. 15 the form of a prism. 



If glass is used in which m = 1.508, 

 then the angle of incidence or the stra- 

 tum of water, which would permit the 

 polarisation to be complete at the sepa- 

 rating surface of the water and the glass, 

 would be exactly 90. 



Polarisation of Light by successive 



Reflexions. 



Although there is only one angle at which 

 incident light can be completely polarised 

 at any surface, viz. an angle whose tangent 

 is equal to the refractive index, yet by re- 

 flecting a ray of light a sufficient number 

 of times, it maybe polarised at any angle 

 of incidence. This property of light was 

 established by Dr. Brewster by the follow- 

 ing experiments made with glass, whose 

 index of refraction was 1.525. 



The numbers in the preceding table 

 were computed by a formula deduced 

 from the observed results for eight re- 

 flexions. 



In the preceding experiments the suc- 

 cessive reflexions were made between two 

 plates of glass placed parallel to each 

 other; but Dr, Brewster obtained the 



same results by arranging the different 

 glass plates so as to form the circum- 

 ference of a polygon, each plate receiving 

 the ray reflected by the one before it. 



It is not necessary that the reflexions 

 should be all made at the same angle. 

 Some of them may be above and some 

 below the polarising angle : for example, 



