The emission spectrum of hydrogen
Some of the most common and readily observable series have been named as shown in this image, where n1 is the ground state and n2 are excited states. The various series are named for the atomic energy level they end on (n1). The series limit where n2 is infinite and n1=1 corresponds to the ionization energy of hydrogen.
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"Hydrogen spectral series."
http://en.wikipedia.org/wiki/Hydrogen_spectral_series
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