Scattering of Light
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Materials can also be investigated examining the light scattered from a material. There are many forms of scattering but the principal ones are as follows;
Rayleigh is elastic scattering from small particles such as atoms or molecules, resulting in scattered radiation that occurs in all directions uniformly. Rayleigh scattering is wavelength dependent with shorter wavelengths being more scattered. It is Raleigh scattering from molecules in the atmosphere which gives rise to the blue sky we see on a fine day. The blue light from the sun striking the upper atmosphere is scattered approximately 10 times more than red light so overhead the blue light is scattered into the eye of an observer while the red light goes largely unscattered and back out into space.
Debye or Mie is an elastic scattering mechanism which occurs from relatively large particles or molecules with dimensions comparable with the wavelength of the incident radiation or larger and the resulting scattered radiation is non-uniform. The effect is not very wavelength dependent. This process gives rise to the white scattered light seen in clouds or fog
Brillouin Scattering is an inelastic scattering mechanism which typically occurs in light scattering from solid materials. The incident radiation wavelength is modified by the energy levels of sound waves or Phonons in the solid material which is typically very small shifts.
Raman is an inelastic scattering mechanism where the frequency of the scattered radiation is changed by the gain or loss of energy which corresponds to energy levels in an atom or molecule. The process is used for many forms of diagnostic analysis. Raman scattering is very weak and is typically much smaller than the Rayleigh scattered light so great care must be taken extract the Raman signal from the Rayleigh signal, particularly for small frequency shifts. For detail see the Raman application notes in the Chemistry section.
Thompson scattering is an elastic scattering mechanism where light is scattered by charged particles. A comparable form of scattering called Compton scattering is an inelastic form of Thompson scattering which occurs when the energy of the incident radiation starts to become comparable to the rest energy of the charged particle. Compton scattering is the main attenuation mechanism of x-rays that provides the contrast in medical x-ray photographs