NOTES 85 



squares of the wave-lengths, are therefore in the ratio 



V = (v-^) 2 



\! 2 V 2 



So that the energy in the length V - v in the second 

 case is 



EV 2 (v _, )= EV 2 



(V - vf x V - v 



The extra energy put in is therefore 



_ v^- EV -^ , 



This is put in by the work done by moving A forward 

 through v against the pressure P. 



Then **> = - 



V v 



EV 



or P = ,y~ 



V v 



If v = o, /. <?. if the source is at rest, 



P = E = energy in length i. 



If v is positive, i. e. if the source is moving forwards, P 

 is greater than P , and if v is negative P is less than P . 

 Neglecting squares and higher powers of z>/V we have 



p _ 



NOTE 3, p. 32. 



THE PRESSURE OF A BEAM INCIDENT NORMALLY ON A 

 PERFECTLY REFLECTING SURFACE 



Let V be the velocity of light, and let E be the energy 

 per cubic centimetre in the beam. Let the receiving 

 surface be moving towards the source with velocity v. 

 Let it be at A at the beginning of a second (fig. 36) 



