The optical course of a light ray when it crosses the interface between two different materials is bent thanks to the phenomenon known as refraction.
An extreme condition of refraction (total reflection) is that in which the ray does not penetrate into the second material but is reflected as if the ray had encountered a mirror. The incidence angle of the ray in which this phenomenon occurs is called the "critical angle".
The magnitude of the critical angle is strictly tied to the refraction indices of the two neighboring materials. If the refraction index of one of the two materials is known, the measurement of the critical angle makes it possible to determine the refraction index of the second material. The refraction index is a physical characteristic specific to each material.
In the case of liquid analysis, it depends on the concentration of the solute.

The Maselli system of analysis measures the refraction index of a liquid by measuring the critical angle, taking advantage of the total reflection phenomenon, and is composed of the following elements:

• light source

• biconvex lens that captures beam from the source

• input lens

• measuring prism with known refraction index

• reflex beam focusing lens

• optical sensor

The beam generated by the source is focused at the prisma-liquid interface being analyzed. The incident rays with an angle inferior to the critical angle penetrate the liquid while those rays with an angle superior to the critical angle are reflected.
The total reflection phenomenon causes the original beam to be partially reflected by the interface and creates an image divided into two parts: one luminous (reflected rays) and one dark (refracted rays). The position of the separation line between light and dark is dependent on the critical angle and therefore of the nd of the liquid being analyzed. Using a CCD optical sensor, the magnitude of the critical angle is detected and a microprocessor processes the data and calculates the nd of the product being analyzed.