Everyone is familiar with the visible beam in a dark movie or vaudeville house or visible sun lays through clouds, especially on a hazy day. In both cases, the rays or beams are visible because of particles of dust and fog in their path, scattering the light. Since the particles are sufficiently large in size, and abundant, enough scattered light comes at an angle from the beam to the observer so that the path of the original beam can be followed.The phenomenon of scattering is basically one of diffraction. However, at the larger particle sizes where the particle diameter is equal to and greater than the wave length(s) of the incident light, a definite proportion of the scattered light is the result of reflection also. Scattered light has, for the most part, the same wave length as the incident light and, therefore, cannot be explained as an ordinary fluorescence.Light scattering has been used for quite some time to distinguish qualitatively between colloidal dispersions and "true" solutions (Faraday-Tyndall effect). Nevertheless, the present trend is to interpret quantitative values connected with the light itself in terms of molecular weight and shape. Useful information is obtained by measuring relative intensity, depolarization, and angular distribution of the scattered light, and analyzing these quantities in terms of the electromagnetic theory of radiation and the kinetic theory of matter.
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