Photon Momentum/Energy: Is It Equal?

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Discussion Overview

The discussion centers on the relationship between the energy and momentum of photons, particularly in the context of photon absorption by atoms. Participants explore the implications of photon energy transfer during electron excitation and the associated momentum transfer to the atom.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • Some participants assert that the energy of a photon is related to its frequency through the equation $$E=hf$$, while its momentum is given by $$p=\frac{hf}{c}$$, leading to questions about how both energy and momentum are transferred during photon absorption.
  • Others argue that when an atom absorbs a photon, the energy required for electron excitation must be matched by the photon's energy, but they also note that the atom must account for momentum transfer, which complicates the absorption process.
  • A participant suggests that if an atom is moving towards the photon, it experiences a blue shift, which could allow for absorption even if the photon’s energy appears lower due to the atom's motion.
  • Some participants discuss the concept of reference frames, indicating that the energy of a photon can vary depending on the observer's frame of reference, complicating the notion of "actual" energy.
  • There is a contention regarding the definition of "atomic momentum," with participants discussing linear and angular momentum in relation to photon absorption and atomic recoil.
  • Participants note that while the momentum change of the atom is equal to that of the photon, the associated change in kinetic energy is relatively small due to the mass difference between the photon and the atom.
  • Some express confusion about terminology, particularly regarding the distinction between orbital and intrinsic momentum of photons.

Areas of Agreement / Disagreement

Participants express differing views on the implications of photon absorption, the definitions of momentum, and the effects of reference frames. There is no consensus on the nuances of these concepts, indicating ongoing debate and exploration.

Contextual Notes

Limitations in the discussion include varying definitions of momentum, the dependence on reference frames, and unresolved questions about the energy transfer mechanisms during photon absorption.

jamie.j1989
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Hi,

Is the energy a photon carries with respect to its frequency the same as that of its momentum energy? My understanding is that it is by the energy relations,

$$hf=E$$
$$E^2=p^2c^2+m^2c^4=p^2c^2$$ for a photon with ##m=0##, frequency ##f## and plank constant ##h## . So we have from both,

$$p=\frac{hf}{c}$$ Where ##p## and ##c## are the momentum of the photon and speed of light respectively.

The issue I have is with photon absorption, as an atom absorbs a photon of frequency ##f## and excites an electron to the next highest state, that state being ##hf## higher than the previous, then how does the photon also transfer momentum to the atom if all its energy is taken by the excitation of the electron?
 
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jamie.j1989 said:
The issue I have is with photon absorption, as an atom absorbs a photon of frequency ##f## and excites an electron to the next highest state, that state being ##hf## higher than the previous, then how does the photon also transfer momentum to the atom if all its energy is taken by the excitation of the electron?
A photon with energy exactly equal to the required delta will fail to excite the atom. The kinetic energy of the recoiling atom needs to be provided.

However, if one has a body with atoms in random thermal motion, some of them will be moving toward the oncoming photons and some will be moving away. This has the effect of smearing out the absorption band -- some atoms will be able to absorb low energy photons and some will be able to absorb high energy photons.
 
But if the atom is moving towards the photon, it see's a blue shift in the photon's frequency, if this blue shifted frequency matches that of delta then surely it can be absorbed even though the energy of the photon is actually less, the extra energy coming from the recoil of the atom in the opposite direction?
 
jamie.j1989 said:
But if the atom is moving towards the photon, it see's a blue shift in the photon's frequency, if this blue shifted frequency matches that of delta then surely it can be absorbed even though the energy of the photon is actually less, the extra energy coming from the recoil of the atom in the opposite direction?
Yes.

Note that the notion of the photon's "actual" energy is not well founded. The energy of a photon depends on your choice of reference frame. There is no one particular energy that is more actual than the others. If one chooses a center-of-momentum reference frame in which the combined momentum of the atom plus photon is zero then the photon's energy can all be used up in increasing the energy level of the electron.
 
jamie.j1989 said:
The issue I have is with photon absorption, as an atom absorbs a photon of frequency ##f## and excites an electron to the next highest state, that state being ##hf## higher than the previous, then how does the photon also transfer momentum to the atom if all its energy is taken by the excitation of the electron?

The entire atom takes up the momentum of the photon. However, since the atom (with its nucleus) is so much more massive, and the momentum of a photon is extremely small, the change in momentum of the atom is negligible.

Zz.
 
ZapperZ said:
The entire atom takes up the momentum of the photon. However, since the atom (with its nucleus) is so much more massive, and the momentum of a photon is extremely small, the change in momentum of the atom is negligible.

Zz.
The change in atomic momentum is equal to the photon momentum.
The associated change in kinetic energy is much smaller than the photon's energy.
 
my2cts said:
The change in atomic momentum is equal to the photon momentum.
The associated change in kinetic energy is much smaller than the photon's energy.

I'm not sure what you mean by "atomic momentum". The change in the orbital angular momentum quantum number is due to the photon's orbital momentum. But it also has a linear momentum k. This is what is absorbed by the atom and caused it to negligibly recoil. In photoabsoroption in solids, the lattice ions absorbed this linear momentum.

Zz.
 
ZapperZ said:
I'm not sure what you mean by "atomic momentum".
Mv, where M is the atomic mass.
The change in the orbital angular momentum quantum number is due to the photon's orbital momentum.
I believe it is due to the photon's intrinsic momentum aka spin.
But it also has a linear momentum k. This is what is absorbed by the atom and caused it to negligibly recoil.
Mv=hk
In photoabsoroption in solids, the lattice ions absorbed this linear momentum.
The entire crystal moves with momentum hk after the absorption.
Indeed, this involves negligible kinetic energy.
 
my2cts said:
Mv, where M is the atomic mass.

I believe it is due to the photon's intrinsic momentum aka spin.

Mv=hk

The entire crystal moves with momentum hk after the absorption.
Indeed, this involves negligible kinetic energy.

Then what are you disputing or arguing against with my post?

This is very puzzling.

Zz.
 
  • #10
Your question about the meaning of "atomic momentum" surprised me but your account is adequate.
One thing, I would call the angular momentum of a photon "spin" and not "orbital" momentum.
 

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