The study of the absorbption/emission of electromagnetic radiation by atoms. Depending on the atomic number characteristic frequency or wavelength are absorbed or emitted. Since each element has a characteristic spectrum of absorbed/emitted wavelengths (spectral signature), atomic spectroscopy allows the determination of elemental compositions even of remote objects (e.g. stars, galaxies, etc.).
Starting from the simple Bohr’s model it is possible to predict quite exactly the frequencies of e.m. radiation selectively absorbed/emitted by all atoms. Depending on the atomic number Z, characteristic frequencies f are absorbed or emitted by atoms corresponding to the electronic transitions from different energetic (quantized) states following the Bohr’s condition: fab=(Eb- Ea)/h, being Ei=-cost∙Z2/(ni)2 the electron energy corresponding to the state/level i (principal quantic number ni). By this way each atomic species has a characteristic spectrum of absorbed/emitted frequencies (atomic spectral signature) so that atomic spectroscopy allows the determination of elemental compositions even of remote objects. By this way the existence of Helium was discovered in the 1968 by Jansen and Lockyer in the Sun photosphere well before its discover on the Earth, and the knowledge of the chemical composition of stars and galaxies was possible well before the end of XIX century. Atomic spectroscopy provides a simple and powerful introduction (through the explanation of the more complex interactions of e.m. radiation with molecules and solid matter) to the fundamental concepts of spectral signature (which is at the base of most of the applications of aerial remote sensing of the Earth’s surface) and atmospheric windows (important for the design of optical sensors devoted to remotely sense Earth’s surface) being moreover propaedeutic to the understanding of methods for the atmospheric vertical sounding based on the concepts spectral lines broadening and related weighting functions.