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B Compton’s experiment vs ordinary mirror

  1. Oct 8, 2016 #1
    Compton hit the electrons in a carbon target with x-rays and measured the changes in the wavelengths of the scattered photons versus their deflected angle. He correlated that with the energy and momentum the incident photon gave to the electron.

    My question is why does this not happen in mirrors? AFAIK there is no red-shift in the much lower energy photons reflected from an ordinary mirror. Is no energy and momentum given to the electrons in the mirror? What is the difference in these cases?
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  3. Oct 8, 2016 #2

    Jonathan Scott

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    As you've already said, the waves in light have much lower energy than X-rays, which also gives them a larger wavelength, so the scale of their interaction is much larger, involving arrays of atoms, and for materials suitable for a mirror the energy is not enough to cause energy level changes in individual atoms which would absorb energy.

    So the light bounces off the mirror as a whole, which does cause a very tiny change in momentum and could in theory give the mirror as a whole some kinetic energy eventually. The effect is far too small to be significant in normal cases, but is the basis of "solar sails" which may be used in future as a means of propelling space probes.
  4. Oct 8, 2016 #3
    Do you think this effect is responsible for the rotation of a Crookes radiometer, or is that view controversial?
  5. Oct 8, 2016 #4

    Vanadium 50

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    What direction would this turn a radiometer?
    In what direction does a radiometer actually turn?
  6. Oct 8, 2016 #5
    I suppose you would expect the white sides to retreat, because the radiation is reflected, not absorbed, imparting more momentum to the vane. I think this is opposite to what is observed, which at least proves that I don't know what is going on.
    BTW, that was an innocent question. Which guideline did I violate?
  7. Oct 8, 2016 #6


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  8. Oct 8, 2016 #7

    Jonathan Scott

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    If you're referring to the guidelines link in Vanadium 50's signature line, that's just for information.

    You did somewhat change the subject of the thread, which could be considered to be against the "hijacking" guideline, but it wasn't far off topic.

    In addition to the above "light mill" article, the Wikipedia article on Crookes radiometer contains some useful information.
  9. Oct 8, 2016 #8
    Thanks for the references.
  10. Oct 20, 2016 #9
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