Emission spectrum of copper. When atoms of an element are excited (for instance by passage through a flame) they return to their state of lowest energy by emitting radiation at specific wavelengths. If this radiation is passed through a spectrometer, a spectrum is produced which displays the element's characteristic emission \lines\". These lines are a unique \"fingerprint\" of an element. In the case of copper, the emission spectrum is complex, with blocks of lines appearing in blue, green and red. Many fainter lines are not visible in this photograph."