CHAPTER XVIII. 



THE ELECTROMAGNETIC THEORY OF LIGHT. 



VELOCITY OF LIGHT IN DIFFERENT MEDIA. 



589. IT has been seen that, on the electromagnetic theory of light, the 

 propagation of waves of light in vacuo ought to take place with a velocity 

 equal, within limits of experimental error, to the actual observed velocity 

 of light. A further test can be applied to the theory by examining whether 

 the observed and calculated velocities are in agreement in media other than 

 the free ether. 



According to the electromagnetic theory, if V is the velocity in any 

 medium, and J the velocity in free ether, we ought to have the relation 



where K Q) /z refer to free ether. 



For free ether and all media which will be considered, we may take //, = !. 

 Also if n is the refractive index for a plane wave of light passing from free 

 ether to any medium, we have from optical theory the relation 



y = n > 



so that, according to the electromagnetic theory, the refractive index of any 

 medium ought to be connected with its inductive capacity by the relation 



I K 



N K\~ 



One difficulty appears at once. According to this equation there ought 

 to be a single definite refractive index for each medium, whereas the pheno- 

 menon of dispersion shews that the refractive index of any medium varies with 

 the wave-length of the light. It is easy to trace this difficulty to its source. 

 The phenomenon of dispersion is supposed to arise from the periodic motion of 

 charged electrons associated with the molecules of the medium, whereas the 

 theoretical value which has been obtained for the velocity of light has been 

 deduced on the supposition that the medium is uncharged at every point 

 ( 577). It is only when the light is of infinite wave-length that the effect 

 of the motion of the electrons disappears. Thus according to the electro- 

 j. 33 



