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Doppler effect review

Review key terms and skills for the Doppler effect, including how to interpret wavefront diagrams.

Key terms

TermMeaning
WavefrontImaginary surface that represents points on a disturbance that all vibrate in unison, such as a ripple that forms from throwing a stone into water.
Doppler effectChange in frequency and wavelength of a wave due to relative motion between the wave source and observer.

Understanding the Doppler effect using wavefront diagrams

Sound waves are longitudinal waves that spread out spherically from their source in all directions, such as from the police car siren in figure 1 below. The distance between two consecutive wavefronts represents the wavelength of the sound wave. The frequency of the wave can be measured by counting the number of wavefronts detected by the observer over a period of time.
Figure 1: When the source and observer(s) are all at rest, the distance between wavefronts is the same for both observers.
For a source and observer with no relative motion, the wavefronts are all centered at the source at all times. Observers on any side will hear the frequency of sound from the source.
When the source and observer are moving relative to each other, the distance between the wave fronts changes depending on where the observer is. For example, if the siren is moving toward the observer on the right, the wave fronts are closer together for observer R and further apart for observer L (Figure 2).
Figure 2: When the source and observer(s) are moving relative to each other, the distance between wavefronts depends on whether the source and observer are getting closer together or further apart..
Keep in mind that the speed of the waves is not changing. The speed depends only on the medium, and the medium isn’t changing. The waves travel at the same speed, but the observed frequency depends on any relative motion between the observer and source.
When the observed frequency changes, so does the wavelength. If the observer and source are moving toward each other, then the frequency increases and the wavelength decreases. In figure 2, observer R on the right sees wave fronts more frequently, so the wave front spacing (or wavelength) is also reduced.
If the observer and source are moving away from each other, then the observed frequency decreases and the wavelength increases. In Figure 2, observer L on the left sees wave fronts less frequently than when the source was at rest, so the wave front spacing is increased. These observations match the
wave equation
when velocity is constant.

Learn more

To check your understanding and work toward mastering these concepts, check out the exercises in this tutorial.

Want to join the conversation?

  • purple pi pink style avatar for user franciscaillaneslecaros
    Why is the doppler effect an "effect" if the frequency and the wavelenght are actually different comparing the waves that are in front of the source and the ones that are left behind?
    (7 votes)
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  • primosaur ultimate style avatar for user harrisonpaulgoodman
    What happens if the source and Observer are both moving?
    (2 votes)
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  • winston baby style avatar for user Daniel
    What if the source of the sound is moving faster or slower (acceleration) relative to the observer?
    (1 vote)
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    • mr pink red style avatar for user Tijani
      Think of an ambulance blaring its siren while you're riding your bike down the street. It is safe to assume that the ambulance is accelerating faster than your riding (if both you and the ambulance are moving toward the same end of the street). The ambulance’s faster acceleration relative to your riding acceleration will cause an increase in the observed frequency because the ambulance will eventually catch up to you (the observer). After all, the wave fronts are still being compressed in front of the ambulance (the source) as it approaches you (the observer).

      On the other hand, if you're a criminal (I hope you're not) that is driving faster than the cops after a bank robbery, your observed frequency of the police siren will decrease because you're driving farther away from the source, which is similar to the source driving away from the observer in the above image from the text.

      I hope this helps.
      (3 votes)
  • leaf red style avatar for user atmacavoy
    Is the frequency that an observer would hear the same everywhere to the direct right of the source? Would it change if the observer was also moving and if so, how?
    (1 vote)
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  • blobby green style avatar for user paseka.manyika
    is it always the case, when we are talking about doppler effects we are mostly reffering to sound only?
    (1 vote)
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  • leaf blue style avatar for user Madeline Valverde
    can someone do an example problem worked out
    (1 vote)
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  • blobby green style avatar for user nolan.jenks
    what happens if the souce and observer are both moving.
    (1 vote)
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  • sneak peak purple style avatar for user Vector Inc.
    The Doppler effect seems to happen when an external observer is moving away from the source that is producing the waves. Our universe is also expanding further and further. Does this mean that since galaxies are becoming more spread-apart from one another, that light waves created by those galaxies are becoming longer the more distant we grow from them? And since red has the longest wavelength of light, would this mean that the waves are getting a redder hue?
    (1 vote)
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    • female robot amelia style avatar for user Johanna
      Yes, the light from distant stars and galaxies is shifting towards the red/ lower energy end of the spectrum when those objects appear to be receding away from us. People can use this “redshift” to measure the universe’s expansion and age.
      (1 vote)
  • blobby green style avatar for user 206036
    What is the definition for observer
    (0 votes)
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