Weegy: Doppler effect is the apparent change in the frequency of a wave because of relative motion between the observer and the source. [ Let's look more closely.
Given a source of a wave and an observer a fixed distance from the source, the wave from the source will arrive at the observer crest after crest after crest. And there will be a fixed time between each crest. This is the period of the wave, or the time it takes for one complete cycle of the wave to occur. (The frequency of a wave is one over the period, and the period of a wave is one over the frequency.) In this scenario, crests will arrive "on schedule" according to the period of the wave. And the frequency of the wave will be observed to be the same as the frequency of the wave generated by the source. Now let's add some relative motion.
If the distance between the source and observer is decreasing, then the observer will be seen to be moving toward the source. And as for the wave, the "next crest" will arrive a bit "sooner" because the observer is "moving to meet" that next crest. The time until the next crest arrives will be shorter for the observer under these circumstances because of the relative motion. The observed wave now has a shorter period, at least according to the observer. That means the frequency of the observed wave is higher because the period is shorter. The source still generates the wave at the original frequency, but the observer sees a wave of higher frequency. If the distance between the source and observer is increasing, the opposite is true.
If an observer is watching, say, a Formula 1 race, Doppler effect will change what is heard from the original sound made by the F1 car. If the observer is in the middle of the straightaway, as the cars come at him, the sound will be higher in frequency, or higher in pitch. The pitch will drop as the car goes by, and the pitch will drop even more after it has passed and is retreating. ]
(More) 