Thomson scattering ( by Joseph John Thomson ) designates the elastic scattering of light ( photons) in free or charged ( as compared to the photon energy ) weakly bonded particles (generally less free electrons). The Thomson scattering is the limiting case of the Compton scattering small photon energies. Both types of scattering based on an elastic collision.
Charged particles are excited by the electromagnetic wave field into coherent harmonic vibrations in the plane of the electric field. Since this oscillation is an accelerated motion, the particle energy in the form of an electromagnetic wave of the same frequency ( dipole ) radiation simultaneously. They say the wave is scattered.
Thomson scattering is a recoilless scattering, that is, there is no momentum transfer from the photon to the electron instead. It occurs only as long as the energy of the incident photons is small enough, i.e., the wavelength of the electromagnetic radiation is much greater than an atomic radius (e.g. soft X rays). At shorter wavelengths, thus higher energy of the recoil of the electron must be considered ( Compton scattering).
This model also applies to free electrons in the metal, whose resonance frequency is due to lack of restoring forces towards zero. Dispersion of bound electrons or all atoms is called Rayleigh scattering.
In practice, one uses ( at not too low densities ), the Thomson scattering as the method for the determination of the electron density (intensity of scattered radiation ) and electron temperature ( spectral distribution of the scattered radiation under the assumption of a Maxwell distribution of the speed).
Where the classical electron radius.
A better approximation for small energies is obtained by expansion of the Klein-Nishina formula:
By a factor of