Spectrum of blue sky, 380 to about 740nanometers (nm).[1] Dips in intensity are observed as dark lines at the wavelengths of the Fraunhofer lines, (e.g., the features G, F, b, E, B). The vertical scale is not actually proportional to intensity.
Solar spectrum with Fraunhofer lines as it appears visually.
In 1802, English chemistWilliam 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 theirwavelengths. He mapped over 570 lines, designating the most prominent with the letters A through K and weaker lines with other letters.[4][5][6][7] Modern observations ofsunlight 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 outerphotosphere. These regions have lower temperatures than gas in the inner photosphere, and absorbs some of the light emitted by it.
The major Fraunhofer lines, and the elements they are associated with, are shown in the following table:
Solar spectral irradiance measured with a calibratedoptical spectrometer. Some of the characteristic Fraunhofer lines and their corresponding elements are indicated for the extendedvisible spectrum (highlighted area in the graph). For photometry and colorimetry, standard measurements are usually carried out in the range 360–830 nm. From these data and for this spectral range, thecorrelated color temperature (CCT) is 5470 K.
A demonstration of the 589 nm D2 (left) and 590 nm D1 (right) emission sodium D lines using a wick with salt water in a flame
The Fraunhofer C, F, G′, and h lines correspond to the alpha, beta, gamma, and delta lines of theBalmer series ofemission 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 thefine-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 inastronomical spectroscopy.
There is disagreement in the literature for some line designations; for example, the Fraunhofer d line may refer to thecyan iron line at 466.814 nm, or alternatively to theyellow 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 therefractive index anddispersion properties of optical materials.
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.
