28 



PRESSURE OF LIGHT 



afforded by two tuning-forks of exactly the same 

 pitch mounted on resonance boxes. If they are 

 both sounded and are both at rest, no beats are 

 heard, since they are in unison. But if, while they 

 are sounding, one of the forks is moved towards an 

 observer, he hears beats, for the waves of the moving 

 fork are shortened and he receives more per second. 

 Though the forks are still emitting the same 

 number of waves per second the hearer is receiving 

 more from one than from the other : the two notes 

 to him are not the same and they beat. 



A' 



FIG. 10. 



Let us now apply this " Doppler Principle " to 

 a source emitting light, or, more generally, radiation 

 of the light kind, whether it is within the range 

 sensible by the eye or not. 



Consider the cases shown in fig. 10. 



In the upper part of the figure suppose that the 

 source A is at rest and that it is emitting waves of 



