In a vacuum, light travels at 300.000 km/s while traveling through water, the speed is about 225.000 km/s, which is 25 % less. In a sapphire, it will only reach 170.000 km/s.
A refractometer is a measuring instrument for the speed of light. The result will not be indicated directly but related to the speed of light in air. This comparison is called the refractive index. The indication, that certain material has a refractive index of 1.5 thus means, that the speed of light travels 50 % faster through the air than through this material.
Who is interested in the speed of light in samples?
The refractive index is a value specific to a material. It depends on the temperature and wavelength (color) of the light. Thus using a refractometer, will enable you to determine the concentration of a material, if temperature and wavelength are known. But it is also possible, that different materials have the same refractive index at various concentrations. Thus a clear determination of liquid substances may only be successful with binary mixtures (Mixtures consisting of two compounds).
In practice, the refractive index determines the mixing ratio also of multi-compound solutions quite exactly and easily as in general only the concentration of one of the components needs to be determined. Thus it is a quantitative measurement. With known compounds, also quality may be determined. For mixtures like for example olive oil or orange juice, the measuring value within a certain range corresponds to a certain quality.
Thus refractometry is quality control of substances.
There is a definite correlation between the refractive index and the composition of many two-compound solutions. The best-known example for such a mixture is a solution of sucrose in water, which has been studied thoroughly. A refractometer can be grated in a way that the value may be indicated directly as dry substance % RTS. For sucrose, this unit is also named Brix (abb. Bx).
The determination of the density of samples can also be done with refractometry. As a rule in optical measurement density and dry substance corresponds to each other.
Refractometers convert the measured refractive indices in concentration or density.