Christian Johan Doppler (1803-1853) was an Australian scientist born in Salzburg who made and important discovery about the effects of sound and light when the sources are moving in relation to the observer. You may find the Doppler effect easier to understand if you consider what happens to the noise of a train as it approaches you and then fades away.
The Doppler principle states that the pitch of a sound is changed, if the object emitting it is moving relative to the observer.
The light emitted, being a moving source, is changed in color, as seen by a relatively stationary observer.
In each case the actual sound and light frequency remains constant. That is to say, diesel locomotive traveling at a constant speed will be producing exactly the same engine noise or “note” whether it is half a mile or a 100 yards from you. But, as it gets closer, a change of tone appears.
The second part of the principle can be applied to the change in color of a moving star.
A more complex example is the Doppler navigational system for aircraft, which required no ground installation. In a typical system four separate beams of microwave energy are radiated from an antenna on the aircraft to the surface of the earth, and some of the energy is reflected back. The frequency of the reflected signal from each beam is shifted by an amount proportional to the plane’s speed. This information is processed by a computer and enables the pilot to fix his position.