the methods used to measure the intensity of scattered visible or ultraviolet light in a given medium in order to determine the concentration, size, and shape of disperse particles in disperse systems. The nature of the scattering of light—that is, the reflection of light by illuminated suspended particles (often called the Tyndall effect)—varies according to the ratio of the size of the disperse particles to the wavelength of the incident light. If the maximum size of suspended particles is less than 0.1 the wavelength, the light scattering is symmetrical in space and is known as Rayleigh scattering. Larger particles produce light scattering that is more intense but irregular, with the scattering being greater in the direction of the incident light beam.

The theory of light scattering is used in measuring both the intensity of scattered light (nephelometry) and the intensity of the transmitted light (turbidimetry), which is reduced because of the scattering. The concentration of the disperse phase, which is used in chemical analysis, can be determined after a calibration has been made by measuring suspensions with known concentrations. The measurement of light-scattering intensity in solutions at various concentrations makes it possible to determine the molecular weights of polymers. The angular dependence of light scattering for large particles and the degree of polarization of the scattered light provide information on the shape of the particles or macromolecules. Nephelometry is also used in studying emulsions and other colloidal systems and in meteorology, marine physics, and some biological investigations.