Atomic, molecular, and optical physics the origin of the index of refraction treated an electron in an atomic system classically according to the model of Paul Drude and Hendrik Lorentz. The theory was developed to attempt to provide an origin for the wavelength-dependent refractive index "n" of a material. In this model, incident electromagnetic waves forced an electron bound to an atom to oscillate. The amplitude of the oscillation would then have a relationship to the frequency of the incident electromagnetic wave and the resonant frequencies of the oscillator. The superposition of these emitted waves from many oscillators would then lead to a wave which
Negative frequency is just a constant whose amplitude level is a measure of the strength of the original "ω" content. And whatever part of "x"("t") that was at frequency zero is changed to a sinusoid at frequency −"ω". Similarly, all other frequencies are changed to non-zero values. As the interval increases, the contribution of the constant term grows in proportion. But the contributions of the sinusoidal terms only oscillate around zero. So "X"("ω") improves as a relative measure of the amount of frequency "ω" in the function "x"("t"). The Fourier transform of formula_1 produces a non-zero response only at frequency "ω". The