250, 251] ELECTROMAGNETIC WAVES. 525 



and the direction of propagation are the same as in insulators, but 

 the ratio of the two fields is 



H 4>7rA\ 



The magnetic field accordingly lags in phase by one-eighth of a 

 period behind the electric, while in an insulator the fields have the 

 same phase. 



251. Reflection of Waves by a Conductor. We shall 

 now consider the effect of a train of plane waves in an insulator 

 striking the plane surface of a conductor which is parallel to their 

 plane. We shall suppose the conductor to extend to infinity in 

 one direction. Let us take the plane # = as the face of the 

 conductor. Let the waves be harmonic in the insulator, for which 

 x < 0, and let the electric force be parallel to the F-axis, the 

 magnetic to the Z'-axis. Then in the wave approaching the con- 

 ductor we have 



VfJU 



In the conductor we have 

 F= 



At the plane x the boundary conditions to be satisfied are 

 that the tangential components of both forces and the normal 

 components of both inductions are continuous. The latter com- 

 ponents being zero we have only the first two conditions to satisfy. 

 There are not enough constants to enable us to satisfy them both, 

 it is accordingly necessary to add to the terms representing the 

 disturbance in the insulator other terms representing a wave 

 travelling in the opposite direction, or a reflected wave. We 

 therefore take 



. 



V/4 V/JL 



