Compton effect
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The Compton effect is the name given to the increase in wavelength which occurs when photons with energies of around 0.5MeV to 3.5MeV, interact with electrons in a material. Because the photons have such high energy, the interaction results in the electron being given enough energy to be completely ejected from its atom, and a photon containing the remaining energy being emitted in a different direction from the original, so that the overall momentum of the system is conserved. If the photon still has enough energy, the process may be repeated. Because of the overall reduction in energy of the photon, there is a corresponding increase in its wavelength. Thus overall there is a slight 'reddening' and scattering of the photons as they pass through the material. This scattering is known as Compton Scattering. In a material where there are free electrons, this effect will occur at all photon energies and hence all wavelengths.
The effect is important in scientific terms because it demonstrates that light cannot be explained purely as a wave phenomenon. Light must behave as if it consists of particles in order to explain the compton effect.
The Compton Effect has on occasion been proposed as an alternative explanation for the phenomenon of the Redshift by opponents of the Big Bang theory, although this is not generally accepted because the influence of the Compton Effect would be noticeable in the spectral lines of distant objects and this is not observed.
See also