Classical waves and the Doppler shift

  • #1
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In the video:
The professor mentions how the wavelength will be the same for a moving observer vs a moving observer for a classical wave like a sound wave. However, how does that explain doppler shift? Don't we observe the effect because a moving observer measures a different wavelength than a stationary observer?
 

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  • #2
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However, how does that explain doppler shift? Don't we observe the effect because a moving observer measures a different wavelength than a stationary observer?

Think carefully about what the moving observer thinks, he does not necessarily think the wavelength has changed. Or read below:



When a sound source moves relative to the medium, then there is a Doppler-shift of the frequency and a Doppler-shift of the wavelength.

When an observer moves relative to the medium, then there is a Doppler-shift of frequency but no Doppler-shift of wavelength, according to the observer. (Wave-train is moving past me extra fast, that is why extra many wave-crests are passing me each second, says the observer)

(If anyone were to look inside the ear of an observer that is moving towards a sound source, he would find there a wave with a 'shortened' wavelength. But that is not the subject now, right?)
 
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  • #3
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(If anyone were to look inside the ear of an observer that is moving towards a sound source, he would find there a wave with a 'shortened' wavelength. But that is not the subject now, right?)
I don't get this. Why will it be a different wavelength?
 
  • #4
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I don't get this. Why will it be a different wavelength?

Motion of the observer changes the pitch that the ear hears. No wind is blowing inside the ear, so the medium is at rest, in the rest frame of the ear.

Hmm maybe it would be better to consider a car that is driving towards a sound source, and the sound waves in the air inside the car.
 
  • #5
A.T.
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However, how does that explain doppler shift? Don't we observe the effect because a moving observer measures a different wavelength than a stationary observer?
Did you look at the Doppler formulas? Does wavelength appear in them?
 
  • #6
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Did you look at the Doppler formulas? Does wavelength appear in them?
Not that I remember. So, Doppler shift happens for normal waves (non-quantum) because of change in frequency because of a moving observer or because of change in wavelength and frequency because of the moving source. Is that correct?
 

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