Light Scattering - Types of Scattering

Types of Scattering

• Rayleigh scattering is the elastic scattering of light by molecules and particulate matter much smaller than the wavelength of the incident light. It occurs when light penetrates gaseous, liquid, or solid phases of matter. Rayleigh scattering intensity has a very strong dependence on the size of the particles (it is proportional the sixth power of their diameter). It is inversely proportional to the fourth power of the wavelength of light, which means that the shorter wavelength in visible white light (violet and blue) are scattered stronger than the longer wavelengths toward the red end of the visible spectrum. This type of scattering is therefore responsible for the blue color of the sky during the day. and the orange colors during sunrise and sunset. Rayleigh scattering is the main cause of signal loss in optical fibers.

• Mie scattering is a broad class of scattering of light by spherical particles of any diameter. The scattering intensity is generally not strongly dependent on the wavelength, but is sensitive to the particle size. Mie scattering coincides with Rayleigh scattering in the special case where the diameter of the particles is much smaller than the wavelength of the light; in this limit, however, the shape of the particles no longer matters. Mie scattering intensity for large particles is proportional to the square of the particle diameter.

• Tyndall scattering is similar to Mie scattering without the restriction to spherical geometry of the particles. It is particularly applicable to colloidal mixtures and suspensions.

• Brillouin scattering occurs from the interaction of photons with acoustic phonons in solids, which are vibrational quanta of lattice vibrations, or with elastic waves in liquids. The scattering is inelastic, meaning it is shifted in energy from the Rayleigh line frequency by an amount that corresponds to the energy of the elastic wave or phonon, and it occurs on the higher and lower energy side of the Rayleigh line, which may be associated with the creation and annihilation of a phonon. The light wave is considered to be scattered by the density maximum or amplitude of the acoustic phonon, in the same manner that X-rays are scattered by the crystal planes in a solid. In solids, the role of the crystal planes in this process is analogous to the planes of the sound waves or density fluctuations. Brillouin scattering measurements require the use of a high-contrast Fabry–Pérot interferometer to resolve the Brillouin lines from the elastic scattering, because the energy shifts are very small (< 100 cm−1) and very weak in intensity. Brillouin scattering measurements yield the sound velocities in a material, which may be used to calculate the elastic constants of the sample.

• Raman scattering is another form of inelastic light scattering, but instead of scattering from acoustic phonons, as in Brillouin scattering, the light interacts with optical phonons, which are predominantly intra-molecular vibrations and rotations with energies larger than acoustic phonons. Raman scattering may therefore be used to determine chemical composition and molecular structure. Since most Raman lines are stronger than Brillouin lines, and have higher energies, standard spectrometers using scanning monochromators may be used to measure them. Raman spectrometers are standard equipment in many chemical laboratories.