SPECIFIC INDUCTIVE CAPACITY 



strain, and the denominator is proportional to the energy possessed 

 per unit velocity. 



If we have two media with different elasticities E x E 2 , but with 

 equal densities, the ratio of the velocities of propagation or the 

 refractive index of sound waves from one to the other will be 



Guided by this analogy, we may regard the energy of electric 

 strain as corresponding to the energy of elastic strain, and the 

 energy of magnetic induction as corresponding to kinetic energy. 

 Since in all transparent media the magnetic permeability is prac- 

 ticallv the same, the energy due to unit induction in the two media 

 is the same, or the media for electric waves correspond to media of 

 equal density for sound waves. The electric modulus is, as we have 



4?r 

 already seen, ^, so that if for two media the dielectric constants 



are K x and K r the analogy suggests that the refractive index should 

 be given by 



the 



If one of the media is air, for which K, = 1, and K is 

 dielectric constant ot the other medium with respect to air, 



Absorbing 

 region ' 



Fio. 83. 



But the analogy is obviously incomplete. In sound waves in 

 gases the elasticity is definite and independent of the periodicity of 

 the waves. A 1 1 waves travel with the same velocity, and the refractive 

 index from one to another is a definite constant. But in light the 

 < itv varies with the periodicity, and we have the phenomena 

 of'diftpenion. The refractive index y. for transparent substances 

 (hcK.iM > in general as the wave-length increases. If, however, a sub- 

 re al)M>ri ;i particular wave-length or a group of wave-lengths, 

 tin- K fr;i( live index in that neighbourhood varies in a manner which 

 is termed anomalous, and the general nature of the connection 

 between /x, the refractive index, and A, the wave-length in air, is 

 hown in Fig. 83. 



