Basic Definitions.

The article presents some basic definitions of Fourier's transforms in Spectroscopy. In the Fourier series, a function h(t) is analyzed into an infinite sum of harmonic components at multiples of the fundamental frequency. If the Fourier series is truncated, and h(t) is approximated by a sum of only a finite number of terms of the Fourier series, then this approximation differs somewhat from h(t). Generally, the approximation becomes better and better as more and more terms are included. The Delta function, also called the impulse function, is a concept which is frequently used to describe quantities which are localized in one point. Even though real physical quantities cannot be truly localized in exactly one point, the concept of delta function is very useful.