PRESSURE OF LIGHT 



when it is moving back it is less than when it is at 

 rest. When it is at rest the pressure is equal to 

 the energy in a unit length of the beam. 



Since, then, the waves are pressing back against 

 the source, the source is pressing forward against 

 them. Or it is giving them forward momentum, 



FIG. ii. 



another way of expressing exactly the same idea. 

 It is giving the waves it sends out momentum, just 

 as certainly as if they were a rush of corpuscles, 

 and the waves carry this momentum forward into 

 space. We need not suppose that the waving 

 material rushes forward. It hands on the momen- 

 tum from layer to layer. We may illustrate this 

 point by hanging a ball so as just to touch one 



