NOTES 87 



The increase in energy is 



. 



V - v V - v 



This can only be accounted for by supposing that 

 there is a pressure P by the waves against the surface, so 

 that work Pv is done against them in moving the surface 

 forward, and we have 



V - v 

 P = V 



V-t> 



If v = o, 



P= 2 E 



or the pressure is equal to the sum of the energies per 

 cubic centimetre in the direct and reflected trains. 

 If the surface is moving, 



E + E' = 

 and P = 



NOTE 4, p. 63. 



THE LIGHT FORCE AT A REFRACTING SURFACE 



In the experiment with the rectangular block of glass, 

 the turning moment is, in all probability, due to the 

 forces introduced at the points of total internal reflection 

 at C and D (fig. 28). 



The forces at the points of entry and emergence, B and 

 E, can, on a very probable assumption, be shown to be 



