Refractive index and structure

The relationship between refractive index and the atomic or molecular structure of a material was considered in Sections 11.1.4 and 11.1.5. Recall that light can be treated as a varying electric field and this interacts with the internal charges on the solid. At the frequency of a light wave, only the electrons respond, and so contribute to the refractive 

In general, strongly bound electrons, trapped at atomic nuclei or in strong chemical bonds, have a low polarisability, and this leads to a low refractive index. Loosely bound electrons, outer electrons on large atoms, or lone-pair electrons, are highly polarisable and so will yield materials with a larger refractive index. 

The refractive index of a solid can be estimated via the Gladstone-Dale formula. It is especially useful for complex oxides, for which the Gladstone-Dale formula can be written  

The assumption underlying the formula is that the refractive index of a complex oxide is made up by adding together the contributions from a collection of simple oxides, oxide 1, oxide 2 and so on, for which optical data are known. The mle works well and usually gives answers within about 5 %. Note, however, that the value obtained is an average 

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