The index of refraction (n) and molar refraction (RD) are indications of the manner in which a molecule interacts with light. The index of refraction is the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v):

n = c/v

This is a dimensionless parameter which ranges between 1.3 and 1.5 for organic liquids. The refractive index is measured using a beam of monochromatic light - typically, the yellow light of the sodium D line (wavelength l = 589.3 nm). Thus, n20D indicates the wavelength used, D, and the temperature, 20'C. Other wavelengths used are the C and F lines of hydrogen l = 656.3 nm and 486.1 nm, respectively) and the G line of mercury ( l = 435.8 nm). Molar refraction, RD, is a function of the density, r, of the medium. The Lorentz-Lorenz equation expresses the relationship between RD, r, and n, based upon electromagnetic theory:

RD =(n^2-1/n^2+2)M/ r

where M is molecular weight and RD has units of volume. (A term related to RD is the specific refraction, which equals RD divided by M.) Rearrangement of this scheme allows evaluation of n:

n = sqrt((M+2 r RD)/(M- r RD))

Molar refraction and refractive indices have many uses. They are often required in confirming the identity and purity of a compound Determination of molecular structure and weight is often aided by these parameters. RD is also used in other estimation schemes, such as in critical properties , surface tension , and the solubility parameter, which is a measure of intermolecular forces . The refractive index, n, is affected by changes in temperature, pressure, and wavelength of radiation. (RD remains nearly constant with changes of temperature and pressure by virtue of the density factor, which is a function of temperature and pressure and. thus offsets these effects.)

The refractive index increases as pressure increases, due to the resulting increase in density. This effect is not as significant with liquids as with gases. Lastly, the refractive index decreases as the wave length increases. For this reason, one cannot compare indices measured at different wavelengths of light.