IB DP Physics: Topic 9. Wave phenomena: 9.5 Doppler effect: Study Notes

9.5 Doppler effect

Essential Idea:
The Doppler effect describes the phenomenon of wavelength/frequency shift when relative motion occurs.


  • The Doppler effect for sound waves and light waves

Applications and Skills:

  • Sketching and interpreting the Doppler effect when there is relative motion between source and observer
  • Describing situations where the Doppler effect can be utilized
  • Solving problems involving the change in frequency or wavelength observed due to the Doppler effect to determine the velocity of the source/observer

Data booklet reference:


When a source of sound and an observer or both are in motion relative to each other there is an apparent change in frequency of sound as heard by the observer. This phenomenon is called the Doppler’s effect.Apparent change in frequency

  1. When source is in motion and observer at rest
    1. when source moving towards observer

    1. when source moving away from observer

Here V = velocity of soundVS = velocity of sourceν0 = source frequency.

  1. When source is at rest and observer in motion
    1. when observer moving towards source

    1. when observer moving away from source and
      V0 = velocity of observer.

  1. When source and observer both are in motion
    1. If source and observer both move away from each other.

    1. If source and observer both move towards each other.

When the wind blows in the direction of sound, then in all above formulae V is replaced by (V + W) where W is the velocity of wind. If the wind blows in the opposite direction to sound then V is replaced by (V – W).


  1. The motion of the listener causes change in number of waves received by the listener and this produces an apparent change in frequency.
  2. The motion of the source of sound causes change in wavelength of the sound waves, which produces apparent change in frequency.
  3. If a star goes away from the earth with velocity v, then the frequency of the light emitted from it changes from ν to ν’.

ν’ = ν (1–v/c), where c is the velocity of light and  where  is called Doppler’s shift.
If wavelength of the observed waves decreases then the object from which the waves are coming is moving towards the listener and vice versa.

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