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BELL SYSTEM TECHNICAL JOURNAL 



dispersion.*^ The index of refraction of an artificial dielectric using f" 

 metallic elements would increase rapidly as the wavelength approached 

 \Y until, at X = \Y the dielectric would be opaque. At still lower wave- 

 lengths, the material would appear to have an index of refraction less than 

 one. 



Fig. 13 — Closely spaced strip coh^ihk lidn comprising copj^er foil strips affixed to 

 cellophane sheets. Juxtaposition of the sheets yielded an effective index of refraction of 

 15. 



Elements, such as rods, which are X/2 long, have a very broad resonance 

 band, and the region of anomalous dispersion in a dielectric utilizing such 



' Anomalous dispersion occurs in optical substances when the frequency of the inciden* 

 light wave approximates one of the vibrational resonance frequencies of the molecule. On 

 the long wavelength side of this resonance region the index of refraction is greater than 

 one and increases rapidly as the resonance wavelength is approached. Dispersion, which 

 is the change of index of refraction with frequency, is therefore very high, but it is the 

 "normal" type of dispersion. At resonance, the absorption of the wave is high and the 

 substance becomes opaque. At still shorter wavelengths, the resonance phenomenon 

 acts to make the index of refraction less than its long wavelength value, often less than one, 

 and the index again varies rapidly with wavelength, but because this is an "abnormal" 

 type of dispersion, it is called the region of anomalous dispersion. 



