Energy in the Electromagnetic Field. 13 



constant value of (1+ l/\/3) 2 and I m i n . a value of (1 — 1/\/3) 2 . 

 The visibility of the fringes is thus stationary along this 

 plane for a considerable distance from the edge. 



Fig. 2. 



Jn advance' 

 of the edge" 



Behind 

 -the edge' 



Intensity-curves at different parts of the Field. 



Passing on to consider the distribution of intensity in any 

 plane Q'P' below the plane y = 0, we have, as remarked 

 above, to add to the effect of the direct and reflected rays a 

 diffraction effect. We shall represent the latter effect by 

 that due to a single source placed at the edge 0, the amplitude 

 of the disturbance due to it at a point in the region bounded 

 by Ox and Oy being given by Sommerfeld's expression * 



kV 



\ /, 7T 



—cosl kr — nt+ -? 

 r \ 4 



>{* 



cos 



++*' 



COS 



4>-f 



(6) 



* Sommerfeld, loc. cit. 



