Refraction of light at the boundary of two media.

Refractometry is a method of investigation of transparent structures, based on the measurement of the refractive index of light (Fig.).
The refractive index (n) is equal to the ratio of the sine of the angle of incidence (a) to the sine of the angle of refraction n = sin α / sin β
(?) is a constant for a specific transparent bodies. [Refer to the figure: 1 - the air; 2 - investigated medium; 3-an incident beam; 4 - refracted ray.]
Usually n is determined at the drop of a monochromatic light (i.e., the rays of light of one wavelength) from the air on the monitoring body, which is at a constant temperature. Used by devices called refractometers. In clinical and sanitary-hygienic laboratories of the refractometry is used for determination of solution concentration (for example, the concentration of protein in the serum), the identification of substances and determine the degree of purity.