A Refractometer is a device used to measure the Specific gravity of liquids by measuring the degree to which light is bent as it enters the liquid sample. For beer brewers this provides a very accurate way to measure the gravity of a wort or beer sample using a very small sample. Typically a few drops of wort or beer are put in the refractometer and the refractive index is read and then converted to specific gravity using a brewing calculator. Measurements are taken as the index of refraction which is a unitless number usually between 1.3000 and 1.7000 expressed to four decimal digits. The refractive index (or index of refraction) of a material is the factor by which the phase velocity of light is slowed in that material, relative to its velocity in a vacuum.
Refractometers are calibrated to measure a known amount of sucrose in a given amount of water. To convert refractive index to a specific gravity, one must perform a calibration adjustment for unfermented wort. Partially or totally fermented wort require a more complex equation due to the presence of alcohol that typically requires both the starting gravity and the current refractive index. Beer brewing software can help you perform this calibration and calculation.