Spectroscopy is the study of how radiated energy interacts with matter. Spectroscopy started with visible spectroscopy and ultraviolet, now most parts of the electromagnetic spectrum are passed through matter to study the interactions. Atoms and molecules have specific responses to radiated energy which is used to identify and quantify them. In atomic spectroscopy the elements are burned in a flame or plasma; the color emitted not only identifies them but quantifies them. Atomic Emission is how we know what a star is made of, by it’s characteristic emission lines.
In UV/Visible spectroscopy researchers are looking for absorption of certain wavelengths of light. Different molecules absorb different wavelengths of visible and UV light in solution. The specific wavelength of light absorbed can indicate whether a specific molecule is present. The amount of light absorbed is directly related to the concentration of the molecule.
In infrared spectroscopy the absorption of light at different frequencies indicates the structure of the molecule probed. Raman spectroscopy, which is in the infrared, observes the shift of wavelength of the scattered light rather than the absorbed light. Raman scattering is a fingerprint of the specific molecules or mixture of molecules. One application of Raman scattering is the identification of counterfeit pharmaceuticals.
X-Rays scattering techniques such as X-Ray diffraction are used to determine the crystallographic structure of materials among other properties. Spectroscopy is performed with wavelengths from microwave to gamma radiation.