THE DOPPLER EFFECT EXPLAINED

To explain the Doppler effect in detail, we will use the example of a train passing.
Let us imagine that a train passes us as we stand next to the railroad tracks, and let us imagine that as it approaches us, there is a long steady blow of the train's whistle. As the whistle blows, sound waves are emitted from the whistle at a constant frequency. If the train were still, the frequency of the waves emitted from the whistle would be the exact frequency that we would hear. Because of the motion of the train, however, this is not the case. Picture in your mind the sound waves being emitted from the train whistle and expanding outward as ever enlarging spheres. When the first wave is emitted, it begins to expand from the point at which it was emitted. Before the second wave is emitted, though, the train has moved forward some. The second wave is then emitted and begins to expand outward. Because of the motion of the train between the two wave emissions, the second wave will be closer to the first wave on the front side of the train, and farther from the first wave on the back side of the train. The train will then move forward some more before the third wave is emitted. Each wave that is emitted ends up being closer to the previous wave on the front side of the train and farther from the previous wave on the back side of the train. In front of the train, the wave fronts are bunched together; behind the train, the motion has spread them out. Now imagine these waves coming toward us as we listen. The waves in front of the train are closer together than they would be if the train were still. They are,however, still moving at normal speed, 343 m/s. Because they are closer together, more of them pass us each second. Saying that more of them pass us each second is equivalent to saying that they pass us at a higher frequency. We therefore hear a higher pitch. After the train passes us, we are then behind it,where the sound waves are spread farther apart. As before, the waves are still moving at 343 m/s. Because they are farther apart, not as many of them pass us each second. We therefore hear a lower frequency sound behind the train. This is our common experience with the Doppler effect.Moving objects seem to produce a higher pitch sound as they approach and a lower pitch sound as they recede.