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When photons reflect off matter, do they always lose momentum to the object? If they lose momentum to the object, that means they lose energy, and so their frequency should decrease. However, when we talk about light hitting objects and being reflected back into our eyes, I never see any mention of decreased frequency. So is it necessary for a photon to lose momentum when colliding and scattering from an object?

(I’m not talking about Compton scattering.)

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    $\begingroup$ It's only necessary that momentum be conserved. Compare the photon's momentum to that of any physical object & you'll see what's going on. But: check out Mossbauer Scattering $\endgroup$
    – Carl Witthoft
    Nov 4, 2015 at 14:56
  • $\begingroup$ @CarlWitthoft Well the photon’s momentum is << an physical object’s. I only ask because my book describes the uncertainty principle by stating photons interact with electrons and inevitably impart momentum to them, and that led me to thinking whether it’s always true that they impart momentum to whatever they impinge. So would I be correct in saying that yes, momentum is imparted to a macroscopic object, but only to a negligible degree? Obviously the macroscopic object won’t be moved much… but how much momentum would the photon lose? Would the transfer greatly impact the photon’s energy? $\endgroup$
    – lightweaver
    Nov 5, 2015 at 13:21

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Now generally, it is true that light incident upon an object is slightly shifted in frequency, but when we come to point of your question there are other factors weighing in.
In these cases it is better to look at the phenomena in Maxwell's wave model of light. When light falls upon a body, it energizes the lattice to vibrate (this can be thought of as transfer of momentum in photon model). Now it so happens that for a certain set of frequencies (dependent on the object) the lattice vibration is almost at the exact same frequency of response, thus the energy is released as another photon of the corresponding frequency, while all the other frequency is taken as heat by lattice vibrations.
So when a range of frequencies are incident upon an object, only a range is vibrated back at almost the same frequency, this we sense as color of the object. Thus the question of shifted frequency is not really in relevance here.

Now back to first part, I said generally because, say in case of Bragg's scattering there is no shift of frequency in the incident x-ray. And this is true even in vision, if the dye reflects exactly 1 wavelength, then there is no frequency shift.
I hope this was kinda satisfactory....

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The only reason I could imagine a frequency shift is

  • fluorescence (ok, this is cheating :-) )
  • ultra slight Doppler effect due to the thermal motion of scatterer/reflector atoms. (at macroscopic scale for many photons and atoms, it's more like an ultra slight frequency blur).
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