Refractometry and interferometry in chemical analysis

In a vacuum, electromagnetic radiation, whatever its frequency (or wavelength), propagates at constant speed.

However, when it has to pass through a material medium, it undergoes various types of interactions that lead to modifications in its propagation, such as :

• attenuation as a function of distance crossed: absorption ;

• modification of its spatial distribution: diffusion ;

• reduction or doubling of its propagation speed: refraction, polarization ;

• variation of propagation speed with frequency: dispersion ;

• frequency change: fluorescence ;

• and other side effects.

In radiation frequency ranges where absorption is of little importance, changes in the propagation speed of radiation can provide information about the nature and/or physical conditions (pressure, temperature) of the medium through which it passes, whatever its state (gas, liquid or solid).

Thus, the absolute refractive index n of a medium relative to vacuum is defined as the ratio c/v of the propagation speed c of monochromatic electromagnetic radiation in vacuum to its speed v in this medium.

Photonic refractometry (ultraviolet, visible and infrared radiation ranges) is the set of refractive index measurement techniques that can play a role in chemical analysis.

Refractometric measurements are only applicable to optically transparent media. However, opaque or strongly tinted gels and liquids can be analyzed by certain methods requiring only a very small quantity of product.