The analysis of spectra, especially light or mass spectra, to determine properties of their source. &diamf3; In light or optical spectroscopy , the spectrum of a light source is analyzed through a spectroscope to determine atomic composition of a substance. In astronomy, phenomena such as red shift can also be analyzed. &diamf3; In mass spectroscopy , a spectroscope is used to determine the composition of ions or charged molecules in a sample. Spectroscopy is also called spectrography.