The study of systems by the electromagnetic radiation with which they interact or that they produce. By convention, spectroscopic properties are expressed in terms of wavelength in vacuo
, not the wavelength within the medium being studied.
PAC, 1985, 57, 105. 'Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)' on page 107 (https://doi.org/10.1351/pac198557010105)