POLARISATION OF LIGHT. 



incidence, and hence this light is called 

 polarised light, because its rays have 

 poles, or sides with different properties. 

 Polarised light is never emitted from 

 any self-luminous body, or from any 

 artificial flame produced by combustion. 

 Whenever it is obtained, it must have 

 previously existed in the state of com- 

 mon light, from which it may be pro- 

 cured in three ways : 



1 . By reflexion from the surfaces of 

 transparent and opaque bodies. 



2. By transmission through a number 

 of plates or planes of uncrystallisedbodies. 



3. By transmission through bodies 

 regularly crystallised, and possessing the 

 property of double refraction. 



CHAPTER II. Polarisation of Light 

 by ReflexionDiscoveries of Mains 

 l)r. Brewster's Law of the Tangents 

 Table of the polarising Angles of 

 bodies Polarisation of Light at the 

 second surfaces of bodies Polarisa- 

 tion of Light at the separating sur- 

 faces of two media By successive 

 Reflexions State of partially po- 

 larised light The polarising angle 

 used to measure refractive powers. 

 In order to [explain the difference be- 

 tween common and polarised light, let 

 A, Jig. 12, be a plate of glass placed at 

 the end of the tube MN, so that a ray 

 of light R A, incident at A, may be re- 

 flected along the axis of the tube M N. 



Fig. 12. 



M 



At the end of another smaller tube N P, 

 which can turn round within MN, place 

 a similar plate of glass, capable of re- 

 flecting a ray A C to the eye at E. 



Let a ray of light R A fall upon the 

 vertical plate of glass A at an angle of 

 incidence of 56, so as to be reflected in 

 the direction AC ; and let this reflected 

 ray A C fall at the same angle of inci- 

 dence of 56 upon a plate of glass C, 

 and be reflected from it to E. Then in 

 the position shown in the figure, where 

 Vhv first reflexion is made in a horizontal 

 plane RAG, and the second in a vertical 

 plane ACF, the ray CE will be so weak 

 as to be scarcely visible, the plate of 

 glass C E having almost no power to 

 reflect the light A C. If we now turn 

 round the tube NP within NM, without 

 shifting the tube MN, and reflector A, 

 the ray C E will become stronger and 

 stronger till it has been turned round 

 90, or so that the plane of reflexion 

 A C E is horizontal like R A C. In this 

 position the light in the beam C E 

 is the greatest possible. If we con- 

 tinue to turn the tube, C E will become 

 fainter and fainter, till after being turned 

 round 90 more, when the plane of re- 



flexion ACE is again vertical, the ray CE 

 will almost cease to be visible. After a 

 farther motion of 90, the ray C E will 

 recover its strength; and by 90 more, 

 which brings the plate C back into its 

 first position, as shown in the figure, 

 the ray C E will cease to be visible. 



From this experiment it clearly fol- 

 lows, that when the upper or the under 

 side of the ray AC is towards or 

 nearest the reflecting plate C, the plate 

 is incapable of reflecting it, whereas 

 when the right or left side of the ray is 

 towards or nearest the reflecting plate, 

 the plate reflects it as it would do com- 

 mon light ; and at intermediate posi- 

 tions intermediate degrees of light are 

 reflected. The ray A C has, therefore, 

 properties different from common light ; 

 and as the common light R A , from 

 which it has been obtained, has suffered 

 no other change but that of reflexion, we 

 are entitled to conclude that light be- 

 comes polarised by reflexion at an angle 

 of 56 from glass. The simple test, 

 therefore, of polarised light is, that it 

 refuses to be reflected by the surface of 

 a transparent body when it is incident at 

 an angle of about 5 6, and in two positions 



