# Why Thomson scattering calls for a size-changing electron?

by Gibby_Canes
Tags: electron, scattering, thomson scattering
 PF Gold P: 1,150 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 $$\frac{8\pi}{3}r_e^2,$$ where $$r_e = \frac{e^2}{4\pi\epsilon_0mc^2}.$$ 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.