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

• wdlang
In summary, the Doppler effect can be used to determine the energy of a photon that has been emitted by an atom. However, the effect is only significant if the initial momentum of the atom is much larger than the momentum of the photon.
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

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.

bcrowell said:
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.

## 1. What is the Doppler effect for atomic spectra?

The Doppler effect for atomic spectra is a phenomenon where the frequencies of light emitted by atoms are shifted depending on the relative motion between the observer and the source of light. This effect results in a change in the wavelength of the light, which can be observed as a shift in the spectral lines of the atom.

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

Yes, the Doppler effect for atomic spectra is a purely realistic effect. It is a result of the laws of classical mechanics and the wave nature of light. It can be observed in various real-world scenarios, such as the redshift of galaxies and the blue shift of stars within our own galaxy.

## 3. How does the Doppler effect for atomic spectra affect the measurement of light from distant objects?

The Doppler effect for atomic spectra can cause a shift in the spectral lines of light emitted by distant objects, which can provide information about their motion and velocity. This effect is often used in astronomy to measure the movement of stars and galaxies.

## 4. Can the Doppler effect for atomic spectra be used to study the behavior of atoms?

Yes, the Doppler effect for atomic spectra is a useful tool for studying the behavior of atoms. By observing the shift in the spectral lines, scientists can gather information about the velocity and movement of atoms. This can be particularly useful in studying the behavior of atoms in extreme conditions, such as in a plasma or in the presence of strong magnetic fields.

## 5. Are there any limitations to the Doppler effect for atomic spectra?

While the Doppler effect for atomic spectra is a well-understood phenomenon, there are some limitations to its application. For example, it is based on the assumption that the source of light is moving in a straight line. In reality, the motion of atoms can be more complex, which can lead to variations in the observed spectral lines. Additionally, the Doppler effect cannot be used to measure the absolute velocity of an object, only the relative motion between the observer and the source of light.

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