The Doppler effect

Another phenomenon associated with waves is called the Doppler effect. Imagine standing near a train track, and a train approaches you with its whistle blowing. The pitch of the whistle you hear though is higher than if the train had been at rest. Furthermore, as the train passes by and moves away from you, the pitch becomes lower.

The pitch of sound waves is associated with the frequency of the wave: the higher the pitch, the higher the frequency. Thus, as the train approaches the frequency of the sound wave you hear is higher, and as it recedes the frequency is lower. This can be pictured by the following diagram.

 

Figure 10.18: The Doppler effect

In this diagram the circles represent, as viewed from above, the crests of adjacent waves. Thus, if you are standing to the right, with the wave source moving towards you, more waves reach you per second, which means that the frequency you hear is higher. Conversely, if you stand to the left, with the wave source moving away from you, fewer waves reach you per second, and the frequency you hear is lower.

The Doppler shift is more difficult to observe in light, since the speeds required are very large. As we will see later on, the Doppler shift plays a crucial role in our understanding of the Universe. It provides very strong evidence that all the galaxies in the Universe are rushing away from us at great speeds: The light reaching us from distant galaxies is shifted to lower frequencies in exactly the same fashion as the train whistle of a receeding train.