Molar and Ionic Refractivity

The molar refractivity is directly proportional to the polarizabilities of the constituent ions of a glass. It can be shown that the molar refractivity, R , is given by the expression  

The molar refractivity of a compound can be calculated from the contributions of each of the constituent ions. The molar refractivity for the compound A By, for example, is given by the sum of the ionic refractivities of the constituent ions, R , times their concentration in the compound, or, in this case  

Since the ionic refractivity depends on the polarizability of the ion, large values are found for the large, low field strength ions such as TT and Pb . Variations in the ionic refractivity explain many of the major trends in the refractive index of glasses. 

Although this method of estimating the molar refractivity works well for many inorganic compounds, it is difficult to apply to oxide glasses. 

Since a typical glass contains from 50 to 80 atomic percent of anions, the ionic refractivities of the anions are very important in controlling the molar refractivity. The polarizabilities of the common anions increase in the order F OFl Cl S Se Te. This 

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