IMPEDANCE CONCEPT AND APPLICATION 



45 



the inverse ratio of the "indices of refraction" of the media. Much 

 could be said, however, in favor of not making such an assumption 

 when formulating the general results since in many applications the 

 permeabilities may be unequal. 



Images 



A few additional interesting results can be obtained for the special 

 case of two semi-infinite homogeneous media having equal propaga- 

 tion constants. If the media are separated by a plane boundary, 

 problems of reflection and refraction can be solved by the method of 

 images. This method is frequently used in electrostatics and one or 



*q 



t-q 



Fig. 6 



two simple examples from that science will serve as an introduction to 

 the later generalizations. 



The field of a point charge g above a conducting plane can be found 

 by assuming another point charge (—5). This "image" charge 

 (Fig. 6) is the same distance below the plane as the actual charge is 

 above the plane, both charges lying on the same perpendicular. The 

 field due to the original charge and to the image charge satisfies the 

 boundary conditions at the conducting plane since it makes the latter 

 an equipotential. This combined field gives the correct resultant field 

 on the same side of the plane as the original charge ; on the opposite 

 side the field is zero. 



If the boundary is the interface between two perfect dielectrics 

 (Fig. 7) with dielectric constants respectively equal to ei and €2, 

 the results are almost equally simple. Above the boundary we have 



