Although the Doppler Effect for sound and light differs, it’s necessary to go through a thought experiment of the Doppler Effect for sound to understand the Doppler Effect for light.
The Doppler Effect is a phenomenon observed whenever a source of waves is moving with respect to an observer. The observer can be stationary or moving with respect to the source. This is best seen here. We will consider the latter situation with a thought experiment below.
Imagine that you are driving on the highway when you notice two police cars in your rearview mirror. The first police car has its siren on and drives by you very slowly. You notice that there is no change in the sound of the siren.
The next police car has its siren on as well, but is traveling much faster. This time you notice that the pitch of the siren increased as the police car approached you and decreased as it passed you. Whereas the pitch of the siren is defined as a measure of the siren’s frequency, frequency is the number of complete sound waves emitted in one second by the siren.
Why did the pitch change when the faster police car passed you? The answer is found in the Doppler Effect.
The Doppler Effect is the effect produced by a moving source (in our case the police car) of waves in which there is an upward shift in frequency when the observer (you) and the source are approaching, and an downward shift in frequency when the observer and the source are moving away. It occurs because the distance between you and the source is changing.
It must be noted that the siren does not change frequency. Rather, the change in frequency is perceived by the observer (you) due to the relative motion between you and the police car.
When you and the police car were approaching each other, the separation between you two decreased. Meanwhile, as you were moving away from each other your separation increased. Considering the frequency of the siren stays the same the same number of waves must fit between you and the source for the same amount of time.
Hence if the separation is small the waves are compressed into the smaller distance. This results in the sound waves reaching the observer more frequently causing him to perceive a high pitch. On the other hand, if the separation is large the observer perceives a lower pitch since the sound waves are reaching the observer less frequently.
Knowing the Doppler Effect allows us to figure out whether the police car is approaching or moving away from us based on the speed of sound in air, our speed, the speed of the police car, and the frequency of the siren.
