Emission spectrum of hydrogen. When atoms of an element are excited (e.g. by heating), they return to their state of lowest energy by emitting radia- tion at specific wavelengths. If this radiation is passed through a spectrometer, a spectrum is prod- uced which displays the element's characteristic emission lines". The lines are a unique "finger- print" of an element. Hydrogen's predominant lines are at 6563 (red), 4861 (cyan) & 4340 angstroms (blue). The many fainter lines are not visible in this photo. Hydrogen atoms can exist in a number of energy states, producing 4 different spectra. This example is known as a "Balmer spectrum"."