Spectroscopy is a branch of science associated with using light to study transitions between energy levels. Not all scientists who use spectroscopy (spectroscopists) are physical chemists, though physical chemists (and physicists) are typically the people who develop new spectroscopic methods and experimentally investigate the details of how light interacts with matter. Data collected in spectroscopic experiments is typically presented as some response of an atomic or molecular system as a function of frequency/wavelength or time. When the response is plotted as a function of frequency/wavelength, it is called a spectrum.