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Light changes frequency depending on

  1. Sep 5, 2009 #1
    velocity of the observer?


    The concept of red-shift seems to suggest, that although the SPEED of light is constant, its wavelength and frequency are not.

    What does this mean in terms of E=hf? The energy the photon possesses increases (in the observer's frame).. IF the observer is moving at a velocity towards it?

    Sure, if you move AWAY at a certain speed from the source of the light you will get red shift, but what if you move towards it? Will you get a blue shift? How fast would you need to be moving towards the light source to see 'gamma shift'. (IS this speed faster than the speed of light?)

    And if the kind of radiation you experience is dependent on your velocity towards or away from the source, how would we determine a light source's "True-wavelength". Is there such thing?
  2. jcsd
  3. Sep 5, 2009 #2


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    Gold Member

    If the lightsource happens to be, say, a sodium lamp then since you know about the physics of sodium lamps you know the frequency of that light in the inertial frame that is co-moving with that source.

    Astronomers are using that all the time, of course. They obtain a spectrum of a star's light, they find the signature spikes of particular elements, and infer the relative velocity of the star from the amount of frequency shift.

    But generally, yeah, the frequency of light is relative. This is analogous to the relativity of velocity. The velocity that you attribute to an object relates to the coordinate system that you are using to map the motion. Objects do not have an intrinsic velocity, and light does not have an intrinsic frequency/wavelength.

  4. Sep 5, 2009 #3


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    Staff: Mentor

    Look up the relativistic Doppler effect equation and calculate it. :smile:

    No, it can't be. But beware that you have to use the relativistic version of the Doppler shift in order to guarantee this. If you try to use the classical Doppler shift you may indeed (wrongly) get v > c.
  5. Sep 5, 2009 #4
    Hi Learnphysics-
    Physicists have been blue-shifting laser photons by backscattering them on oncoming electron beams for many years. For example, an IR laser photon (energy ~ 1 eV) backscattered on a 1-GeV electron (gamma ~ 2000) will yield a backscattered photon with an energy of about 16 MeV. The laser photon energy boost is about 4 gamma2. The cross section for photon-electron (Thomson) scattering is about 0.66 barns (0.66 x 10-24 cm2). The kinematics for the (Compton) scattering of photons on electrons is given in
    The kinematics of the accelerated electron laser photon backscattering application is given in

    [Added] See also kinematic formulas for Compton scattering in
    Bob S
    Last edited: Sep 5, 2009
  6. Sep 5, 2009 #5
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