Nephelometry/Catalog of methods in biophysics

A true solution or solvent appears homogeneous in transmitted light, but colloidal solutions appear opalescent. The cause of this phenomenon is the scattering of light, which is a complex phenomenon that includes refraction, reflection and bending of light depending on the size of the particles. If the indices of refraction of the particles and the solvent are sufficiently different, light scattering is applied to the extent that the light is observable in a direction other than the direction of propagation. This phenomenon is called the Tyndall effect. The method called nephelometry deals with measuring the intensity of scattered light coming from a colloidal solution perpendicular to the direction of the light beam that caused the scattering of light. The measurement can be used at constant particle size to determine their concentration and at constant concentration to determine their size.

The dimensions of colloidal particles are smaller than or comparable to the wavelength of visible radiation and therefore cannot be observed in an ordinary microscope. Thanks to the Tyndall phenomenon, however, they can be made visible in the so-called ultramicroscope. It is an ordinary optical microscope with side illumination of the solution. Particles of colloidal dimensions appear in it as glowing and moving points on a dark background. Particles from about 5 nm can be seen. This is not a real image of the particle, but a bending phenomenon caused by the particle, so it is not possible to determine the size or shape of the particle in the ultramicroscope, but only its presence.

Determining the shape and size of particles such as viruses, protein molecules and nucleic acid molecules is made possible by the electron microscope, which is based on the fact that a beam of slow electrons under a constant voltage has the properties of a light beam.

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