Refractive Index is an optical characteristic of a substance and the dissolved particles on it.

The refractive index of a substance is strongly influenced by the temperature and the wavelength of the light used to measure it. Therefore, care must be taken to control or compensate for temperature differences and wavelength. The refractive index measurements are usually reported at a reference temperature of 20°C (68°F), which is considered to be room temperature.

Refractive index is defined as the ratio of the speed of light in a vacuum to the speed of light in a substance. A result of this property is that light will "bend", or change direction, when it travels through a substance with a different refractive index. This is called refraction.

When passing from a material with a higher to lower refractive index, there is a critical angle at which an incoming beam of light can no longer refract, but will instead be reflected off the interface between the two substances. This is calledtotal internal reflection.

The critical angle can be used to easily calculate the refractive index according to the equation:

                                                                                                                                                                                     sin (Θcritical) = n2 / n1

Where n2 is the refractive index of the lower-density medium, n1 is t he refractive index of the higher-density medium.

A digital refractometer passes an LED light through a prism in contact with the sample. AN image sensor determines the critical angle at which the light is no longer refracted through the sample. Specialized algorithms then apply temperature compensation to the measurement and convert the refractive index to the specified parameter.