The importance of the frequency change is that if the frequency increases, then the time it takes for one complete cycle (oscillation) is less. The opposite is true for a light source that is moving away from you or that you are moving away from. If you approach the light source or the light source approaches you, the frequency of the light will appear to increase (notice that the waves in the front are closer together than in the rear). The relativistic Doppler effect is the change in frequency, wavelength and amplitude1 of light, caused by the relative motion of the source and the. In the second part, you can see that source 'S' is moving to the right and the light waves are shifted (they look as though they are being compressed in the front and dragged in the rear). In the top part of fig 7 you can see a stationary light source is emitting light in all directions. This being said, let's look a fig 7 below. This Doppler shift also affects light (electromagnetic radiation) in the same manner with one critical exception the shift will not allow you to determine if the light source is approaching you or if you are approaching the source and vice versa for moving away. I'm sure that at some point in our lives, all of us have imitated the sound of a passing car or passing police car we imitated the Doppler Shift. One last example is the change in tone you hear when a police car passes you with its siren on. You can hear a definite shift in the sound of the car as it passes where you are standing.

Another example occurs when cars race around a racetrack. When the train passes you, you can hear the whistle tone change to a lower note. As the train approaches, you hear the whistle tone as a high note. Think about an approaching train blowing its whistle. When a sound source approaches you, the frequency of the sound increases and likewise, when the sound source moves away from you, the frequency of the sound decreases. There is a Doppler Shift associated with sound waves that you should recognize easily.

A good place to start in understanding the Doppler effect would be to first look at sound waves. Also, the amplitude of the shift is dependent on the speed of the source (or the speed of the receiver). The direction of the shift is dependent on whether the relative motion is traveling towards you or away from you (or vice versa). The Doppler Effect basically says that there is an observed frequency shift in electromagnetic waves due to motion.

Relativistic doppler shift