Refractive index measurement is a measurement of the speed of light in a medium. The speed of light (usually denoted by c) is 299 792 458 m/s in a vacuum. In other media, the speed of light is lower, and the refractive index (R.I.) of a medium is how much slower the light's speed is in the medium.
The detection of liquid concentrations by optical means is not new. The law of refraction was mathematically formulated first by Ibn Sahl in 984 but not known in Europe. Instead, its discovery misattribution goes to the Dutch astronomer and mathematician Willebrord Snellius (Snell), who rediscovered the law and published it in 1621. The first laboratory instrument to accurately measure liquids' refractive index was developed by Ernst Abbe in 1874.
This technical paper, courtesy of Vaisala K-Patents, is a technical explanation and understanding of the refractive index.