MAGNETISM AND LIGHT 



327 



A train of waves of circularly polarised light may be regarded 

 as consisting of circular disturbance, represented by a radius of 

 given length travelling round each point in the path and with the 

 same angular velocity for each radius, the ends of the radii being 

 on a revolving spiral. A corkscrew turned round counter-clockwise 

 without advancing exactly represents such a train of waves with 

 counter-clockwise rotation travelling towards its point, and if it 

 be laid lengthwise on a mirror its reflection in the mirror repre- 

 sents the transmission of a train with clockwise rotation. Let (a), 

 Fig. 242, represent the right-handed corkscrew as seen from above, the 

 thick lines representing the upper part. Then if it is turned round 



(a,) 



M 



B 1 



M 



FIG. 242. 



A 1 



(b) 



FIG. 243. 



counter-clockwise as seen from A, a given displacement will travel 

 forward from A to B. If (b) represents its reflection in the 

 mirror MM, a clockwise rotation as seen from A' will send a 

 given displacement forward from A' to B'. 



If the two displacements in (a) and (b) are combined, they will 

 always give an up-and-down displacement perpendicular to the 

 plane of the paper, or a plane-polarised disturbance. 



But now suppose that while the speed of rotation of the two 

 is the same, one travels faster forward than the other, that is, has 

 a longer wave length, or is a steeper spiral. .Thus in Fig. 243 let 

 ACDB represent the faster-moving and longer wave, AEFG the 

 slower and shorter. At the instant represented they coincide at A, 

 and as the revolution is at the same rate for both spirals they 

 combine now and always to give at A a disturbance perpendicular 

 to the paper. At this instant they coincide again at G, and 

 combine now and always to give at G a disturbance in a line which 

 is turned round in the direction of the circular motion which has the 

 greatest forward velocity. That is, there is rotation of the plane 

 of polarisation. 



