Doppler effect

Learn about the 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.

When the vehicle was stationary, the wavefronts remained centered on the vehicle as they spread out. But now, since the vehicle is moving to the right, each subsequent wavefront is NOT centered on the same point that the previous wavefront was.

We can think of the wavefronts as a record of where the siren was at different times. The siren was at the center of any given wavefront when it emitted that wavefront. Since the vehicle is moving to the right, the center of each subsequent wavefront also moves to the right.

Regardless of where a given wavefront is emitted from, the wavefront spreads out symmetrically from there—it doesn't "know" that the siren is moving. As a result, the wavefronts get stretched out behind the vehicle, and bunched up in front of the vehicle. Or, more precisely, the wavelength increases for an observer behind the vehicle, and decreases for an observer in front of 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?

Khan Academy video wrapper

See video transcript

Additional notes about the Doppler effect

The Doppler effect is NOT the result of wave speed changing. The speed that each wavefront travels forward is the same whether the source is moving or not.
Similarly, the "true" frequency that's emitted by the source is NOT changing. For example, an observer in the vehicle would not notice any difference in frequency wether the vehicle was moving or not.
We looked at the case of a moving source and stationary observer. However, an observer would also notice a Doppler shift if they were moving relative to a stationary source. For example, if you were riding on a train and passed a stationary siren, you'd observe the same Doppler effect as if you were stationary and the siren drove past you at the same speed.
The greater the speed at which the source and observer are moving relative to each other, the greater the observed shift in frequency will be.
The Doppler effect does not only occur with sound waves—it occurs with electromagnetic (light) waves too. Since the speed of light is so high, we don't observe the Doppler effect with light waves in a everyday sense like we do with sound waves. However, Doppler shift of light has many essential applications, including Doppler radar and measuring the expansion of the universe.

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