HOW LIGHT EXERTS PRESSURE 2/ 



receive the same number per second as an observer 

 at B. The two observers will, therefore, hear notes 

 of the same pitch, since pitch depends solely on 

 the number of waves entering the ear per second. 

 In the lower part of the figure let the engine be 

 travelling from left to right as it whistles. Sup- 

 pose that at one moment it is at C and a moment 

 later while it has sent out four waves let it have 

 moved to D. The first wave will have travelled to 

 A in one direction and to B in the other the same 

 distances as before, for the velocity of the waves is 

 not altered by the travel of the engine. But the 

 fourth wave will have just issued when the engine 

 is at D, so that in the direction of motion the four 

 v/aves are crushed up into the shorter length AD 

 and the observer at A receives more per second, 

 while, behind, they are opened out and the observer 

 at B receives fewer per second. The pitch of the 

 note is therefore higher at A and lower at B. 



The Doppler effect is easily observed by putting 

 a pitch-pipe into one end of a rubber tube several 

 feet long. The experimenter blows through the 

 open end and whirls the tube in a horizontal circle 

 round his head, the pitch-pipe moving round the 

 circumference of the circle. A hearer outside the 

 circle notes a rise in pitch as the pipe approaches 

 him and a fall as it recedes, though to the experi- 

 menter the pitch is the same all the time. 



Another interesting illustration of the effect is 



