Topics - Refractive Index
Classically defined the refractive index of a medium is the ratio of the velocity of light through a vacuum to the velocity of light through the medium. It is also the ratio of the sine of the angle of incidence of a beam of light entering the medium to the sine of the angle of refraction. The refractive index changes with the wavelength of light with which it is measured and, thus, the wavelength of the light must be quoted when the refractive index is reported. The electromagnetic theory of light shows that the refractive index of a medium is related to the dielectric constant of the medium by the Lorentz-Lorentz Equation. The measurement of refractive index is used in liquid chromatography detection in a number of ways, e.g., the Fresnel method, the Christianson, method, the Interferometric method are but a few of the measuring techniques used in refractive index detectors. Refractive index is a bulk property
of the mobile phase and, thus, detection based on refractive index has the inherent limited sensitivity of bulk property detector, viz
10-6 to 10-7 gram per ml. As the refractive index and dielectric constant of a medium are related, the properties of the dielectric constant detector are very similar to those of the refractive index detector, although the methods of measurement are quite different. Because the refractive index detector tends to have a universal response, and despite its limited sensitivity, it finds use in the detection of those substances (fatty acids, alcohols, sugars etc) that are not easy to detect by other means.