24 PRESSURE OF LIGHT 



we halve the length of wave, keeping the height 

 and depth the same, we require four times as much 

 energy in the same length to change the shape. It is 

 easily seen that with three waves in the same length, 

 we should have nine times as much energy, and 

 generally the energy in given length for waves of 

 the same height is in inverse proportion to the 

 square of the wave-length. 



The dependence of the energy of motion on 



FIG. 8. 



the length of wave is seen by considering fig. 8, 

 where two trains of waves of equal height and 

 depth are represented. The lower train has half 

 the wave length of the upper train. Let them be 

 supposed to travel at equal speed from left to right. 

 Any particle, such as D, moves up and down as the 

 waves pass by it and through an equal excursion 

 in each case. But the particle has to make twice 

 as many excursions in the same time in the lower 

 train, and must therefore move with an average 



