Photons and the Doppler effect

  1. I am currently capturing and analyzing spectra. My analysis software includes allowances for Doppler shifts resulting from relative radial motion between source and observer, and also Earth’s rotation. Results are accurate .

    The basic introduction to the Doppler effect generally starts with waves in water with a static and then a moving wave generator which very clearly shows wave length changes.
    Sound waves – similar demonstrations with sirens etc,. – also very clear effect.

    When it comes to light I have a ‘blind spot’.
    Using the example of an Hα photon (say 6563 Å) emitted from a hydrogen atom (keep it simple - nearby star, ignoring Earth’s rotation).
    Situation 1:
    No relative radial motion. Photon measured by observer at 6563 Å.

    Situation 2:
    Source ‘static’, observer moves towards or away from approaching photon. Observer measures blue or red shift.

    Situation 3:
    Source moves towards ‘static’ observer. Here I need HELP. This photon, once emitted, is traveling at the speed of light – this being independent of the motion of the source. I am currently unable to see how the beginning wavelength would be measured differently by the observer.


  2. jcsd
  3. Drakkith

    Staff: Mentor

    What exactly do you need help with? A moving source is identical to a moving observer. From the observers point of view, they cannot tell whether they are moving towards the source, or whether the source is moving towards them. (And in reality either view is correct)
  4. mfb

    Staff: Mentor

    This is easier to understand if you let the star emit 1 pulse of light per second. Within that second, the star moves a bit towards you (or away from you), so the next pulse will star closer to you (further away from you), and you get the pulses with a higher (lower) frequency.
    The same stays true if you look at oscillations of electromagnetic waves instead of discrete light pulses.
  5. Thanks for your responses. I do not deny current wisdom.The point I have tried to make is, in the specific situation noted, I will measure the wavelength of a single photon. How could this photon, having departed the star, and moving at light speed, be affected by any motion of that star?
  6. mfb

    Staff: Mentor

    It comes from that star. How could it not be affected by properties of the star?
  7. Thanks. I would be very happy to understand what properties of the star effect the departed photon such that there is a change in the beginning wavelength.
  8. Drakkith

    Staff: Mentor

    It's not a matter of the star's properties affecting the photon. From the star's point of view the photon is the same wavelength. It is only because of the relative motion between the star and the observer that the photon is blue/redshifted.
  9. mfb

    Staff: Mentor

    That's not the right time-order. In our frame of reference, the photon is always blueshifted. It is not a process that happens magically after the photon was emitted.

    See my light pulse analogy, I think that is easier to understand.
  10. Go back to the siren example. The speed of sound in air is the same. Why does the tone change as the ambulance is moving towards or away from you?
  11. Same theory as Doppler radar police use to determine automobile speeds.

    Three things affect the observed frequency of a photon:

    Change in gravitational potential between emitter and receiver,

    Relative motion between emitter and receiver [your specifically stated issue]

    Expansion or contraction of space between emitter and receiver [as over cosmological distances]
  12. I sincerely thank all my new found colleagues for their replies.


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