86 PRESSURE OF LIGHT 



and at B at the end of the second. Then during that 

 second it receives the radiation in the length, CA = 

 V + v. 



The radiation reflected will be crowded up into the 

 length, BD, where AD = V, and BD therefore = V - v. 



If A! is the wave-length of the incident beam, and X 2 

 that of the reflected beam, there are the same number of 

 wave-lengths in CA and in BD. Then 



Xp V -v 



\, ~ V + v 



The perfect reflection requires that the resultant dis- 

 turbance at the reflecting surface shall always be zero. 



V 



B 



FIG. 36. 



Hence the amplitudes in the incident and reflected 

 waves must be equal and at the surface opposite. If, 

 then, E' is the energy per cubic centimetre in the reflected 

 train, 



?:_v /v^/ 



E A 2 2 \V - v. 

 The energy in length, V + v, of the incident train 

 cross-section i is 



E(V + v) 



That in length V - v of the reflected train is 





