84 PRESSURE OF LIGHT 



If the corpuscles could be supposed to have spinning 

 energy as well as translational energy, and equal to it in 

 amount, and if further they could be supposed to give 

 up this spinning energy,on absorption, the total energy 

 per c.c. would be mV 2 , or would be equal to the 

 pressure. 



NOTE 2, p. 29. 



THE PRESSURE DUE TO WAVES ISSUING NORMALLY 

 FROM A SURFACE 



We may obtain the value of the pressure thus 



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



V 



A 



A' 



FIG. 35. 



energy per cubic centimetre emitted in a beam issuing 

 perpendicularly from a square centimetre when the source 

 is at rest. Now let the source be moving forward with 

 velocity v, as in fig. 35, where A represents the source 

 at the beginning of the second, B the point reached by 

 the waves from it at the end of the second, and A' the 

 position of the source at the end of the second. If A X 

 is the wave-length for the source at rest, A 2 that for the 

 moving source, the number of waves occupying V in the 

 first case are crushed into V - v in the second case, or 



X 2 V - v 



We assume that the heights or amplitudes are the same. 

 The energies in equal lengths, being inversely as the 



