A refractometer is an instrument used for measuring concentrations of aqueous solutions. It requires only a few drops of liquid.
When liquid is used in the refractormeter the light changes direction; this is called refraction. Refractometers measure the degree to which the light changes direction. A refractometer takes the refraction angles and correlates them to refractive index (nD) values that have been established. Using these values, you can determine the concentrations of solutions.