Classical waves and the Doppler shift

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Discussion Overview

The discussion revolves around the Doppler shift in classical waves, particularly sound waves, and how it is perceived by moving observers versus stationary observers. Participants explore the implications of observer motion on wavelength and frequency, questioning the relationship between these concepts and the Doppler effect.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants question how a moving observer perceives wavelength in relation to the Doppler shift, suggesting that the observer may not perceive a change in wavelength despite the shift in frequency.
  • Others argue that when a sound source moves relative to the medium, both frequency and wavelength experience a Doppler shift, while a moving observer only perceives a frequency shift.
  • A participant proposes that the motion of the observer affects the pitch heard, implying that the medium remains at rest in the observer's frame of reference.
  • There is a suggestion to consider a scenario involving a car moving towards a sound source to clarify the effects of motion on wave perception.
  • Some participants reference the Doppler formulas, questioning whether wavelength is included in these equations and discussing the relationship between frequency and wavelength shifts in different scenarios.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between observer motion and the perceived changes in wavelength and frequency. The discussion remains unresolved, with multiple competing interpretations of the Doppler effect being presented.

Contextual Notes

Participants highlight the complexity of the Doppler effect, noting that assumptions about the medium and the observer's frame of reference may influence their understanding. There are unresolved questions regarding the mathematical representation of the Doppler shift and its implications for wavelength perception.

Phys12
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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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Phys12 said:
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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jartsa said:
(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?
 
Phys12 said:
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.
 
Phys12 said:
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?
 
A.T. said:
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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