The Maxwell's relation in electromagnetic theory states that refractive index is equal to the square root over the di-electric constant. This relation holds for some simple gases.But more generally,it does not.My question is why this is the case. Eugene Hecht says that this is because n (refractive index) is a function of frequency.But I do not understand.After all, di-electric constant is also frequency dependent quantity. What I think is that for di-electric substances,due to the very structure of the substances,the frequency is only measurable at low frequencies.So,that when we try to match the calculated n (observed in visual range of EM spectrum) with the di-electric constant =Limit (frequency--->0) [K(w)] (where K is the di-electric constant and w is the frequency),the two values do not match. Please confirm if I am correct.