522 Mr. E. Edser on the Reflexion of Electromagnetic 



waves are identical with those that would be produced i£ the 

 mirror were stationary. 



Finally, let the mirror move tangentially with velocity v 

 in the direction vertically upwards. Then, the condition 

 that the tangential force on a unit positive charge, placed 

 near the mirror and moving with it, shall vanish, gives 



Ej + E^O. 



Since the component of the force on a unit N pole, resolved 

 normally to the mirror, must vanish at the surface of the 

 mirror, we must have 



-(E 1 sin(9-E 2 sin6'') = 0, 



c 



.*. sin#'= — sin#, 

 and the ordinary law of reflexion is again obeyed. 



Oblique Reflexion — Electric Field Parallel to 

 Plane of Incidence. 



12. Let the axes of reference, the angles of incidence and 

 reflexion, and the motion of the mirror be defined as in § 6, 

 but let the electric field of the wave-trains be parallel to the 

 intersection of the wave-front and the plane of incidence, the 

 sign of fhe electric field being chosen so that when 6 — 6 f = 0, 

 the positive direction of the electric field is from South to 

 North. Further, let the positive direction of the magnetic 

 field of either wave-train be measured vertically downwards. 

 Let Ex denote the force that would be exerted on a stationary 

 unit positive charge by the incident waves, and let E 2 denote 

 the corresponding force due to the reflected waves. Then a 

 unit positive charge, placed just in front of the mirror and 

 moving with it, will be acted on by a force comprising the 

 component 



E x cos 0+E, cos 0' + -(Ei-E,) 



c 



in the direction from South to North, and therefore tangential 

 to the mirror, and the component 



E x sin 0+E 2 sin 6' 



from East to West, and therefore normal to the surface of the 

 mirror. Since the tangential component of this force must 

 vanish at the surface of the mirror, we have 



E 1 (ccos^ + v) + E 2 (ccos6' / -v)=0. . . (38) 



