# Is the Doppler effect for atomic spectra a purely realistic effect?

#### wdlang

i am now studying the Doppler effect in a thermal atomic gas

If an atom travels in velocity v along x direction

at some time, it emits a photon in some direction

the momentum of the emitted photon can be well approximated with the free one

thus the momentum of the atom after the emission is well defined

We can then determine the energy of the photon with the energy conservation law

In this procedure, the Doppler effect apprarently depends on the MASS of the atom

However, the fomulae given in the book does not depends on the MASS but only on the velocity of the atom

Related Quantum Physics News on Phys.org

#### bcrowell

Staff Emeritus
Gold Member
I replied to your post in the relativity area. I'm fairly new here myself, so I could be wrong about the local customs, but I think it's not a good idea to cross-post something like this in two areas at once, because it could end up wasting people's time by making redundant replies -- they won't realize that a parallel discussion is going on in another area.

#### wdlang

I replied to your post in the relativity area. I'm fairly new here myself, so I could be wrong about the local customs, but I think it's not a good idea to cross-post something like this in two areas at once, because it could end up wasting people's time by making redundant replies -- they won't realize that a parallel discussion is going on in another area.

hmm, i did the calculation according to my method and the result is similar to that given by the book but with a slight difference which is proportional to the recoil energy of the atom, which depends on the mass of the atom (inversely proportional to the mass actually). As long as the initial momentum of the atom is much larger than the momentum of the photon, this term can be neglected.

"Is the Doppler effect for atomic spectra a purely realistic effect?"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving