Debye extended the theory to include random coil polymers. Gans in 1925 derived scattering formulas appropriate for spheres of finite size, and in 1947 P. Rayleigh's basic theory has been extended by several authors. The remaining scattering arises from fluctuations in particle density discussed above and was first analyzed theoretically by A. This leads to fixed phase relations and destructive interference for most of the scattered light. There is strong correlation between the motion of neighboring particles. The breakdown of Rayleigh's second assumption-that of independent particles-occurs in all liquids. The Tyndall effect is the reason clouds are white (the water droplets become larger than the wavelengths of visible light). Such scattering is called the Tyndall effect and results in a nearly wavelength-independent (that is, white) scattering spectrum. Colloidal suspensions provide systems in which the scattering particles are comparable to or larger than the exciting wavelengths. Under some circumstances of interest, both of these assumptions fail. Rayleigh's derivation of his scattering equation relies on the assumption of small, independent particles. Under the assumption that the scattering in fluids is from particles much smaller than the wavelength of the exciting light, Lord Rayleigh derived in 1871 an equation, shown below, In solids this process is obscured by scattering from defects and impurities. Scattering from entropy or temperature fluctuations is called Rayleigh scattering. Under rather special circumstances, however, these entropy or temperature fluctuations can propagate this has been observed in super-fluid helium and in crystalline sodium fluoride at low temperatures, and is called second sound. This is because entropy fluctuations in a normal fluid are not propagating and do not, therefore, have a characteristic frequency unlike sound they do not move like waves through a liquid-instead, they diffuse. In contrast to the pressure fluctuations due to sound waves, these entropy fluctuations do not generate scattering at sharp, well-defined wavelength shifts from the exciting wavelength rather, they produce a broadening in the scattered radiation centered about the exciting wavelength. The second kind of inelastic scattering studied in fluids is due to entropy and temperature fluctuations. See also: Absorption of electromagnetic radiation Fluorescence Luminescence Phosphorescence If the delay is longer than a microsecond or so, the process may be called fluorescence and mechanisms involving very long delays (seconds) are usually termed phosphorescence. Mechanisms in which there is a measurable delay between absorption and reemission are usually termed luminescence. True “scattering” refers only to those processes which are essentially instantaneous. Scattering processes may be divided according to the time between the absorption of energy from the incident beam and the subsequent reradiation.

The scattering process in these wavelength regions consists of acceleration of the electrons by the incident beam, followed by reradiation from the accelerating charges.

Light in the visible, infrared, or ultraviolet region interacts primarily with the electrons in gases, liquids, and solids-not the nuclei. Maxwell in the nineteenth century that accelerating electric charges radiate energy and, conversely, that electromagnetic radiation consists of fields which accelerate charged particles. See also: Light Meteorological optics Radio-wave propagation At longer wavelengths, scattering of radio waves determines their characteristics as they pass through the atmosphere.

In the visible wavelength region, scattering of light produces the blue sky, red sunsets, and white clouds. At slightly longer wavelengths, the scattering of x-rays provides the most effective means of determining the structure of crystalline solids. In the short-wavelength, high-energy regime in which electromagnetic radiation is most easily discussed by means of a particle description, these processes are termed photon scattering. All electromagnetic radiation is subject to scattering by the medium (gas, liquid, or solid) through which it passes. The process in which energy is removed from a beam of electromagnetic radiation and reemitted with a change in direction, phase, or wavelength.

Scatter radiation