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Fraunhofer lines are a set of spectral lines named after German physicist Joseph von Fraunhofer. The lines were originally observed as dark features(absorption lines) in the optical spectrum of the Sun.
Fraunhofer was not the first to observe these lines, but he did do the most extensive study of the phenomenon. In 1814, he began a systematic study and careful measurement of the wavelengths of these features. He mapped over 570 lines, and designated the principal features with the letters A through K, and weaker lines with other letters. Modern observations can detect many thousands of these lines. The increase is mainly due to the improvement of technology. It was later discovered that a chemical element was associated with each set of spectral lines, and that the dark lines in the solar spectrum were caused by the absorption of those elements in the upper layers of the Sun.
The Fraunhofer lines C, F, G’, and h correspond to the alpha, beta, gamma and delta Balmer series of emission lines of the hydrogen atom. The D1 and D2 lines form the sodium doublet. The center wavelength(589.29 nm) is given the designation letter “D”. This designation for this line is given to all of the transitions between the ground state and the first excited state of the alkali atoms as well. The D1 and D2 lines correspond to the fine splitting of the excited states. Do not confuse this with the transition that is the P-state of the alkali, not the higher D-states. There is some ambiguity for some line designations: the Fraunhofer d-line may refer to the cyan iron line or to the yellow helium line, also the e-line can refer to the spectral lines of both iron and mercury. To resolve ambiguities that arise in usage, some Fraunhofer line designations are preceded by the element with which they are being associated.
Since Fraunhofer lines have well defined wavelengths, they are often used to characterize the refractive index and dispersion properties of optical materials and thus may be used in two different reference frames in astronomy.