HOW LIGHT EXERTS PRESSURE 29 



the length ABC. In the lower part of the figure 

 suppose that the same source is moving forwards, 

 and, to take a very extreme case for simplicity of 

 calculation, suppose that it moves at half the speed 

 of the waves, so that while the front of the wave 

 travels to C the back is just issuing from A'. The 

 height and depth of the waves will be the same, for 

 they depend only on the temperature of the source, 

 which is the same in the two cases. We must 

 suppose, too, that the energy given out by the light 

 sources is the same in the two cases. But in the 

 latter case the energy in the wave is twice as great 

 in the half length, since it is four times as great in 

 an equal length. We can only account for this 

 extra energy by supposing that the wave presses 

 like a compressed spring against the surface from 

 which it starts, and that we supply the extra energy 

 in moving the source forwards against the pressure. 



If the source is moving backwards it is easily 

 seen that the waves will contain less energy than 

 that emitted from the light source and that the 

 difference between that emitted from the source 

 and that existing in the medium is to be accounted 

 for by the work done by the wave pressure in 

 pushing the source back. 1 



When the theory is worked out fully it is found 

 that the pressure is greater when the source is 

 moving forward than when it is at rest, and that 

 1 Note 2, p. 84. 



