Lyman-alpha line
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In physics, the Lyman-alpha line is a spectral line of hydrogen in the Lyman series, emitted when the electron falls from the n=2 orbit to the n=1 orbit, where n is the principal quantum number. Its wavelength of 121.6 nanometres, corresponding to a frequency of 2.47 × 1015 hertz, places the Lyman-alpha line in the far ultraviolet part of the electromagnetic spectrum.