How does a refractometer work?

The refractometer can be used to measure the salinity and specific gravity of water, plasma protein in blood sample, and specific gravity of urine. A refractometer measures the refractive index of a sample. The refractive index is the ratio of the speed of light traveling in a vacuum to the speed of light traveling in the sample. The refractometer utilizes the physics of light to find the refractive index of the liquid, since light bends when passing through one medium to another, in this case, the liquid sample to the prism. How the light bends is proportional to the refractive index of the sample and the refractive index of the sample can be read on the scale of the refractomenter. The salinity of the sample can be calculated from the salinity vs. refractive index plot (y=5585.5x-7446.2). Based on Frank J. Millero and Alain Poisson’s measurements, the density as a function of salinity and temperature was derived (at 1atm). The equation is the following: ρ= ρ0 + AS + B(S^3/2) + CS, where A, B, and C are functions of temperature, S is the salinity, and ρ0 is the density of water.

Most of the time, an Abbe refractometer is used to measure the refractive index. Alternatively, the refractive index can also be measured by using diffraction grating. Note that this apparatus can be made by yourself with materials such as a diffraction grating, laser pointer, transparent tray, clips for hanging cloth, tracing paper, and tape. Here is the link for how to measure the refractive index using this method: http://www.pl.euhou.net/docupload/files/Excersises/WorldAroundUs/IndexOfRefraction/IndexOfRefraction.pdf