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### Course: High school physics (DEPRECATED)>Unit 10

Lesson 4: Doppler effect

# Doppler effect

If a wave source is moving relative to an observer, the observer perceives a change in the frequency of the wave. This phenomenon is called the Doppler effect.
Let's explore the Doppler effect using a familiar example: a siren on a vehicle. The model below shows a stationary vehicle with a siren that's emitting sound waves. The sound waves are represented as wavefronts emerging from the siren. Two observers, one on the left (L) and one on the right (R), hear the sound from the siren.
Use the model to complete the statements.
The observer on the left hears a frequency which is
the frequency emitted by the siren.
The observer on the right hears a frequency which is
the frequency emitted by the siren.

Since the siren is not moving, all wavefronts remain centered on the siren as they spread out. So, both observers hear the sound at its "true" frequency—the frequency that is emitted by the siren.
Now let's consider what happens when the vehicle is moving to the right. The siren still emits the same frequency sound wave as before. But now, we see a difference in the pattern of wavefronts around the vehicle.
Use the model to complete the statements.
The observer on the left hears a frequency which is
the frequency emitted by the siren.
The observer on the right hears a frequency which is
the frequency emitted by the siren.

Due to the siren's motion away from the observer on the left, wavefronts reach them less frequently than they did when the siren was stationary. As a result, they hear a lower frequency sound than what the siren is actually emitting.
Due to the siren's motion towards the observer on the right, wavefronts reach them more frequently that they did when the siren was stationary. As a result, they hear a higher frequency sound than what the siren is actually emitting.
Check out the animated models of both scenarios below.
The scenario above includes two different observers, on different sides of the moving siren. But often, there is just one observer—you!—and a wave source passes you. In this case, you're like the observer on the right (R) as the source moves toward you, and you're like the observer on the left (L) as the source moves away from you.
Watch and listen to the recording of the moving car horn below. Can you hear the frequency change as the car passes the observer?