Why Thomson scattering calls for a size-changing electron?

In summary, Thomson scattering, which involves the scattering of X-ray radiation by free electrons, only works for wavelengths comparable to the size of the electron. This led to the assumption that the size of the electron must change depending on the wavelength of the incoming light. However, the Compton Effect provides a better explanation for this phenomenon without imposing constraints on the electron. Thomson scattering is not directly related to the size of the charge distribution of the electron and can be calculated using the Thomson cross-section formula, which assumes that the photon energy is much greater than the binding energy of the atom. The Compton effect, on the other hand, shows the relationship between the change in wavelength of gamma radiation and the angle of scattering.
  • #1
Gibby_Canes
21
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Why Thomson scattering calls for a "size-changing" electron?

From my limited understanding of Thomson scattering, it only works for wavelengths comparable to the size of the electron. Because scattering was observed at a variety of wavelengths, it was assumed that the size of the electron must change when rays of different wavelengths were scattered off of it. One of the merits of the Compton Effect was that it described this phenomenon without imposing unlikely constraints on the electron.

But I don't understand why? Why does the scattering only work for certain wavelengths? And how exactly does this transfer into the idea of an electron of variable size?
 
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  • #2


In classical optics, the scattering of a wave off a barrier depends on the size of the barrier and the incoming wavelength.
Knowing the wavelength, you should be able to work out the size of the barrier - it is a common teaching lab experiment.
The equation should work for any wavelength - it's just that the ones that are comparable to the size of the barrier give easy to measure patterns.

When you use the same model to find the "size" of an electron, you get different values depending on the wavelength of the incoming light. So it appears that different kinds of light see different sizes for the electron ... or: the classical model has a flaw in it ;)
 
  • #3


Gibby_Canes,

what you describe does not seem correct. Can you post a reference where you read it?

Thomson scattering is a term referring to scattering of X-ray radiation by (almost) free electrons; the X-rays are supposed to be comparable or lower than the size of the atoms, not electrons.In practice, Thomson let X-rays scatter off the air molecules and calculated effective scattering cross-section for such scattering (for Poynting energy).

However, this cross-section is not connected directly with the size of the charge distribution of the electron. Point-like electron of zero radius still has non-zero Thomson cross-section

[tex]
\frac{8\pi}{3}r_e^2,
[/tex]

where

[tex]
r_e = \frac{e^2}{4\pi\epsilon_0mc^2}.
[/tex]

In fact, the Thomson derivation is for situation where the wavelength is much greater than the electron.

The Compton effect is related, but was about something different - it shows the relation between the change in wavelength of the gamma radiation and the angle of scattering.

Neither Thomson or Compton's calculations use the idea that electron has definite non-zero size.
 
  • #5


I would like to clarify that Thomson scattering does not necessarily call for a "size-changing" electron. Rather, it is a consequence of the classical theory of electromagnetism, which describes the interaction between electromagnetic radiation and charged particles. In this theory, the size of the electron is not considered, and the electron is treated as a point particle.

However, when considering the scattering of radiation by an electron, the wavelength of the radiation must be comparable to the size of the electron in order for the scattering to occur. This is because the scattering process involves the interaction of the electric field of the radiation with the electric field of the electron. If the wavelength of the radiation is much larger than the size of the electron, the electric field will not be able to interact with the electron effectively and therefore, no scattering will occur.

Therefore, the concept of a "size-changing" electron is not necessary to explain Thomson scattering. It is simply a consequence of the classical theory of electromagnetism and the physical properties of the electron. The Compton effect, which considers the electron as a particle with a finite size, provides a more accurate description of the scattering process, but it is not required to understand Thomson scattering.
 

1. Why is the size of the electron important in Thomson scattering?

The size of the electron is important in Thomson scattering because it affects the strength of the scattered radiation. A larger electron has a larger electric field, resulting in a higher scattering cross-section and thus a stronger scattered signal.

2. How does the size-changing electron affect the scattering angle in Thomson scattering?

The size-changing electron can affect the scattering angle in Thomson scattering by changing the wavelength and frequency of the scattered radiation. This can result in a different scattering angle compared to a fixed-size electron.

3. What is the relationship between the size of the electron and the intensity of the scattered radiation in Thomson scattering?

The size of the electron is directly proportional to the intensity of the scattered radiation in Thomson scattering. A larger electron results in a stronger scattering signal, while a smaller electron results in a weaker scattering signal.

4. How does the size-changing electron affect the polarization of the scattered radiation in Thomson scattering?

The size-changing electron can affect the polarization of the scattered radiation in Thomson scattering by changing the orientation of the electric field. This can result in a different polarization state compared to a fixed-size electron.

5. Is the size of the electron constant in Thomson scattering?

No, the size of the electron can vary in Thomson scattering depending on the energy and wavelength of the incident radiation. This is due to the wave-particle duality of electrons, where they can behave like particles with a defined size or waves with a varying size.

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