500 BELL SYSTEM TECHNICAL JOURNAL 



same at all frequencies in the electrical range and is equal to the square 

 of the optical refractive index. This must mean that the only polari- 

 zable elements of consequence in CeHe are electrons which are capable 

 of polarizing as readily in the visible spectrum, where the refractive 

 index is measured, as at lower frequencies where dielectric constant is 

 measured. The refractive index in the visible spectrum provides the 

 means of determining the magnitude of electronic polarizations, for 

 other types of polarization are usually of negligible magnitude when 

 the frequency of the impressed field lies in the visible spectrum. For 

 materials having only electronic polarizations the dielectric properties 

 are very simply dependent upon the chemical composition and the 

 temperature, and are independent of frequency in the electrical 

 frequency range. In many materials, however, there are also other 

 polarizations which can form at low frequencies but not at high ; these 

 are characterized by more complex dielectric behavior. 



Atomic Polarizations 



Included among the polarizations which may be described as in- 

 stantaneous by comparison with the order of magnitude of the periods 

 of alternation of the applied field in the electrical frequency range are 

 those arising from the displacement of the ions in an ionic crystal 

 lattice (such as rock salt) or of atoms in a molecule or molecular lattice. 

 In some few materials, for example the alkali halides, sufficient study 

 has been made of the infra-red refractive index to provide data on the 

 atomic polarizations, but for most substances little is known about 

 them. What is known has in part been inferred from infra-red absorp- 

 tion spectra and in part from the infra-red vibrations revealed by 

 studies of the Raman effect. 



Atomic polarizations are distinguished from electronic polarizations 

 by being the part of the polarization of a molecule which can be at- 

 tributed to the relative motion of the atoms of which it is composed. 

 The atomic polarizations may be attributed to the perturbation by the 

 applied field of the vibrations of atoms and ions having their character- 

 istic or resonance frequencies in the infra-red. Atomic polarizations 

 may be large for substances such as the alkali halides and other in- 

 organic materials, but are usually negligible for organic materials. 

 The exact value of the time required for the formation of atomic 

 polarizations is unimportant in the electric range of frequencies with 

 which we are primarily concerned. The essential thing is that atomic 

 polarizations begin to contribute to e(or n^) at frequencies below 

 approximately 10^^ seconds — that is, in the near infra-red and that 

 below about 10^" cycles per second, where the optical and electrical 



