The Doppler Effect (in general)

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Discussion Overview

The discussion revolves around the Doppler effect as it applies to various types of waves, including sound and light. Participants explore the relationship between the observed frequency of waves and the relative motion of the source and observer, questioning how frequency changes as the source approaches and then recedes from the observer.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that the observed frequency increases as the source approaches and decreases as it recedes, while others question why this occurs if the relative velocity remains constant.
  • One participant cites a source stating that the observed frequency varies continuously as the source approaches and recedes, leading to confusion about the nature of relative velocity during these intervals.
  • There is a discussion about whether the Doppler effect applies differently when the source is moving directly toward the observer versus at an angle, with some arguing that the frequency changes continuously in the former case and others suggesting it is constant until the source passes.
  • Participants note that the intensity of sound may also change as the source approaches and recedes, although this point is not universally agreed upon.
  • Some contributions highlight the distinction between the speed of light being invariant and the relative motion of the source affecting the observed frequency, particularly in the context of sound waves.
  • There is a mention of the radial component of the rate of change of the position vector of the source concerning the observer, emphasizing the complexity of the situation.

Areas of Agreement / Disagreement

Participants express differing views on how the Doppler effect operates, particularly regarding the constancy of relative velocity and the nature of frequency changes. There is no consensus on whether the observed frequency changes continuously or only at specific points of motion.

Contextual Notes

Some participants highlight the importance of the reference frame and the radial component of motion, indicating that assumptions about the source's trajectory can significantly affect interpretations of the Doppler effect.

  • #61
I wasn't talking about the 'radial' or 'transverse' direction----we can have these components of relative velocity whatever direction(north,south,east) the motion is in.

I was actually referring to the importance of 'position' of source and observer in the analysis.Besides,as long as we know the relative velocity,and the direction of it,it doesn't matter whether it's north or south,what matters is the direction w.r.t each other (moving towards or moving away)--we can choose north-south directions as we please.

The line I'm referring to is on the page given below,in the third last paragraph of the section 'Analysis'.

http://en.wikipedia.org/wiki/Doppler_effect
 
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  • #62
Time reversal?Iwould wwant to know more on it
 
  • #63
Urmi Roy said:
I wasn't talking about the 'radial' or 'transverse' direction----we can have these components of relative velocity whatever direction(north,south,east) the motion is in.

I was actually referring to the importance of 'position' of source and observer in the analysis.Besides,as long as we know the relative velocity,and the direction of it,it doesn't matter whether it's north or south,what matters is the direction w.r.t each other (moving towards or moving away)--we can choose north-south directions as we please.

The line I'm referring to is on the page given below,in the third last paragraph of the section 'Analysis'.

http://en.wikipedia.org/wiki/Doppler_effect
That wiki paragraph is referring to the fact that in a more complete analysis one must take the travel time of the signal into account (from source to observer) in order to determine what the observer actually observes at any given time. To do that requires knowing the position of the source at the time of emission, not just its direction of motion.
 
  • #64
Thanks,Doc Al,I understand now.
 
  • #65
Waves farther from the source lag behind those closer to it.So if you have positive radial velocity greater than the speed of sound you hear the song played backwards
That's time reversal
 

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