The study of the absorption and emission of light and other radiation by matter. Involves the splitting of light (or more precisely electromagnetic radiation) into its constituent wavelengths (a spectrum), which is done in much the same way as a prism splitting light into a rainbow of colours. More recently, the definition has been expanded to include the study of interactions between particles such as electrons, protons, and ions, as well as their interaction with other particles as a function of their collision energy.