Emission spectrum of helium. When atoms of an element are excited (for instance by heating), 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'. The lines are a unique 'fingerprint' of an element. Helium's predominant lines (right to left) are at 6678 angstroms (red), 5875 (yellow), 5015 & 4921 (cyan) and 4713, 4471 & 4388 angstroms (blue). Many fainter lines are not visible in this photograph.

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