In 1802, English chemist
William Hyde Wollaston[2] was the first person to note the appearance of a number of dark features in the solar spectrum.[3] In 1814,
Joseph von Fraunhofer independently rediscovered the lines and began to systematically study and measure their
wavelengths. He mapped over 570 lines, designating the most prominent with the letters A through K and weaker lines with other letters.[4][5][6] Modern observations of
sunlight can detect many thousands of lines.
The Fraunhofer lines are typical spectral absorption lines. Absorption lines are narrow regions of decreased intensity in a spectrum, which are the result of photons being absorbed as light passes from the source to the detector. In the Sun, Fraunhofer lines are a result of gas in the Sun's atmosphere and outer
photosphere. These regions have a lower temperatures than gas in the inner photosphere, and absorbs some of the light emitted by it.
Naming
The major Fraunhofer lines, and the elements they are associated with, are shown in the following table:
The Fraunhofer C, F, G′, and h lines correspond to the alpha, beta, gamma, and delta lines of the
Balmer series of
emission lines of the hydrogen atom. The Fraunhofer letters are now rarely used for those lines.
The D1 and D2 lines form a pair known as the "sodium doublet", the centre wavelength of which (589.29 nm) is given the designation letter "D". This historical designation for this line has stuck and is given to all the transitions between the ground state and the first excited state of the other alkali atoms as well. The D1 and D2 lines correspond to the
fine-structure splitting of the excited states.
The Fraunhofer H and K letters are also still used for the calciumII doublet in the violet part of the spectrum, important in
astronomical spectroscopy.
There is disagreement in the literature for some line designations; for example, the Fraunhofer d line may refer to the
cyan iron line at 466.814 nm, or alternatively to the
yellow helium line (also labeled D3) at 587.5618 nm. Similarly, there is ambiguity regarding the e line, since it can refer to the spectral lines of both iron (Fe) and mercury (Hg). In order to resolve ambiguities that arise in usage, ambiguous Fraunhofer line designations are preceded by the element with which they are associated (e.g., Mercury e line and Helium d line).
Because of their well-defined wavelengths, Fraunhofer lines are often used to specify standard wavelengths for characterising the
refractive index and
dispersion properties of optical materials.
See also
Abbe number, measure of glass dispersion defined using Fraunhofer lines
Gustav Kirchhoff (1859)
"Ueber die Fraunhofer'schen Linien" (On Fraunhofer's lines), Monatsbericht der Königlichen Preussische Akademie der Wissenschaften zu Berlin (Monthly report of the Royal Prussian Academy of Sciences in Berlin), 662–665.
Gustav Kirchhoff (1859)
"Ueber das Sonnenspektrum" (On the sun's spectrum), Verhandlungen des naturhistorisch-medizinischen Vereins zu Heidelberg (Proceedings of the Natural History / Medical Association in Heidelberg), 1 (7) : 251–255.