Waves by a moving perfect Reflector. 



707 



its energy- density divided by c * ; hence the normal com- 

 ponents of the increment of momentum cancel, and the 

 tangential components make up 2e/c . vBt . sin d. The pro- 

 duction of this momentum is to be assigned to the mechanical 

 forces. Thus, up to the first order of v/c the forces exerted 



i*r 



_„_— 



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/ \\ 







"'' \ 



JZ\ 



?;St 









/ R ^-. 





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on the reflector are made up of the thrust of the incident 

 beam, the back thrust of the reflected beam and a tangential 

 traction on the surface in the direction of travel of the radia- 



tion and equal to 2e~sin#, which may in a general statement 

 c 



be apportioned between the incident and reflected beams. 



This tangential traction was missed in the previous esti- 

 mate. It is easy to see in a general way whence it comes, in 

 the case of polarization perpendicular to incidence. The 

 electric density Esin0/4ire a , receding with velocity v, then 

 constitutes a current which is acted on by the tangential 

 magnetic field E/c. Again, the thrusts along the two beams 

 may not balance tangentially. But further scrutiny in detail 

 must be deferred until a happier time. 



Cambridge, Oct. 7, 1914. 



* Cf. for developments a lecture, " On the "Dynamics of Radiation,'' 

 Report of Fifth Int. Math. Congress, Cambridge, 1912, vol. i. pp.197-216. 



