22 PRESSURE OF LIGHT 



spiral spring. The waves contain energy. If we 

 compress them into a shorter length we have to 

 put more energy into them, somewhat as we have 

 to put more energy into the spiral spring when we 

 crush it up. The ends of the spring are pressing 

 outwards, and if we compress it we do work 

 against the outward pressure, and so we put extra 

 energy into the spring. Similarly when we com- 

 press waves into a shorter length we put more 

 energy into them, and therefore they, like the 

 spring, must be pressing outwards at each end or 

 must exert a pressure against any surface which is 

 moved so as to crush them up. 



Wave energy is of two kinds, that due to the 

 shape of the waves and that due to the motion of 

 the material waving. 



As an illustration of the energy due to shape 

 and its dependence on length of wave, we will take 

 the special case of zigzag waves on a stretched 

 cord, which we will suppose of india-rubber. 



Let AB, fig. 7(0), be the cord, stretched to some 

 extent. Let C be the middle point, and let D and 

 E be the middle points of AC and CB. Suppose 

 that D is drawn up a small distance DM, and that 

 E is drawn down an equal small distance EN, as 

 in fig. ?(), so that C remains in the original line. 

 Then work is done in pulling D and E out of 

 position, or energy is put into the cord in chang- 

 ing its shape into the zigzag wave form ADCEB. 



