HOW LIGHT EXERTS PRESSURE 3! 



edge of a long table as in fig. II. If we give 

 a sharp blow with a hammer to the opposite edge 

 a wave of pressure travels through the table, a 

 wave of momentum, put in by the hammer, which 

 is given up by each part of the table to the next 

 and which finally arrives at the ball and presses it 

 out. 



Let us now follow the course of a limited train 

 of waves travelling out perpendicularly from a 

 source A to a receiving surface B, fig. 12. When 

 the waves start from A, as at a, they are pressing 



a 



FIG. 12. 



against A, and A is pressing against them, that is, 

 A is pouring forward momentum into them. They 

 rush out carrying the momentum with them, and 

 when they are clear of A, z. e. when A has ceased 

 to pour out momentum, it is no longer pressed. 

 The momentum is now being carried through mid 

 space as at c. Let the waves finally strike the 

 surface B. If they are absorbed there, that is, if 

 B is a quite black surface, the waves cease and as 

 they cease they deliver their momentum up to B. 

 As they take the same time to pass into B that 

 they took to issue from A, and as they give up in 

 that time all the momentum which they received 



