View source for Fraunhofer lines

{{Short description|Spectral lines in the Sun's spectrum}}
[[Image:Spectrum of blue sky.svg|thumb|right|upright=1.6|Wavelengths of the visual spectrum, 380 to about 740 [[nanometers]] (nm).<ref>{{cite book | title = Biology: Concepts and Applications | author = Starr, Cecie | publisher = Thomson Brooks/Cole | year = 2005 | isbn = 978-0-534-46226-0 | url = https://archive.org/details/biologyconceptsa06edstar| url-access = registration | page = [https://archive.org/details/biologyconceptsa06edstar/page/94 94] }}</ref> 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 '''Fraunhofer lines''' are a set of [[spectral line|spectral absorption lines]]. They are dark [[absorption line]]s, seen in the [[optical spectrum]] of the [[Sun]], and are formed when atoms in the [[solar atmosphere]] absorb light being emitted by the [[solar photosphere]]. The lines are named after German physicist [[Joseph von Fraunhofer]], who observed them in 1814.

==Discovery==
[[Image:Fraunhofer lines.svg|thumb|left|upright=1.6|Solar spectrum with Fraunhofer lines as it appears visually.]]
In 1802, English chemist [[William Hyde Wollaston]]<ref name="eb">Melvyn C. Usselman: [http://www.britannica.com/EBchecked/topic/646649/William-Hyde-Wollaston William Hyde Wollaston] Encyclopædia Britannica, retrieved 31 March 2013</ref> was the first person to note the appearance of a number of dark features in the solar spectrum.<ref>William Hyde Wollaston (1802) [http://rstl.royalsocietypublishing.org/content/92/365.full.pdf+html "A method of examining refractive and dispersive powers, by prismatic reflection,"] ''Philosophical Transactions of the Royal Society'', '''92''': 365–380; see especially p. 378.</ref> In 1814, [[Joseph von Fraunhofer]] independently rediscovered the lines and began to systematically study and measure their [[wavelength]]s. He mapped over 570 lines, designating the most prominent with the letters A through K and weaker lines with other letters.<ref>{{cite book|last=Hearnshaw|first=J.B.|title=The analysis of starlight|date=1986|publisher=[[Cambridge University Press]]|location=Cambridge|isbn=978-0-521-39916-6|page=27}}</ref><ref>Joseph Fraunhofer (1814 - 1815) [https://books.google.com/books?id=2-AAAAAAYAAJ&pg=PA203 "Bestimmung des Brechungs- und des Farben-Zerstreuungs - Vermögens verschiedener Glasarten, in Bezug auf die Vervollkommnung achromatischer Fernröhre"] (Determination of the refractive and color-dispersing power of different types of glass, in relation to the improvement of achromatic telescopes), ''Denkschriften der Königlichen Akademie der Wissenschaften zu München'' (Memoirs of the Royal Academy of Sciences in Munich), '''5''':  193–226; see especially pages 202–205 and the plate following page 226.</ref><ref>{{Cite book
  | last = Jenkins  | first = Francis A.
  | last2 = White   | first2 = Harvey E.
  | title = Fundamentals of Optics
  | url = https://archive.org/details/fundamentalsopti00jenk  | url-access = limited  | edition = 4th
  | publisher = [[McGraw-Hill]]
  | date = 1981
  | page = [https://archive.org/details/fundamentalsopti00jenk/page/n37 18]
  | isbn = 978-0-07-256191-3
  }}</ref> Modern observations of [[sunlight]] can detect many thousands of lines.

About 45 years later, [[Gustav Kirchhoff]] and [[Robert Bunsen]]<ref>See:
*  Gustav Kirchhoff (1859) [https://books.google.com/books?id=CMgAAAAAYAAJ&pg=PA662"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) [https://books.google.com/books?id=uksDAAAAYAAJ&pg=RA1-PA251 "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.</ref> noticed that several Fraunhofer lines coincide with characteristic [[emission spectrum|emission lines]] identified in the spectra of heated chemical elements.<ref>{{cite journal 
 |author= G. Kirchhoff
 |title=Ueber die Fraunhofer'schen Linien
 |journal=Annalen der Physik
 |volume=185 |issue=1 |pages=148–150 |date=1860
 |doi=10.1002/andp.18601850115|bibcode = 1860AnP...185..148K |url=https://zenodo.org/record/1423666
 }}</ref> They inferred that dark lines in the solar spectrum are caused by [[absorption (electromagnetic radiation)|absorption]] by [[chemical element]]s in the solar atmosphere.<ref>{{cite journal 
 |author= G. Kirchhoff
 |title=Ueber das Verhältniss zwischen dem Emissionsvermögen und dem Absorptionsvermögen der Körper für Wärme und Licht
 |trans-title=On the relation between the emissive power and the absorptive power of bodies towards heat and light 
 |journal=Annalen der Physik
 |volume=185 |issue=2 |pages=275–301 |date=1860
 |doi=10.1002/andp.18601850205|bibcode = 1860AnP...185..275K |url=https://zenodo.org/record/1423668|doi-access=free}}</ref> Some of the observed features were identified as [[telluric contamination|telluric lines]] originating from absorption by [[oxygen]] molecules in the [[Earth's atmosphere]].

==Sources==
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 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:

[[File:Solar spectral irradiance.svg|thumb|right|upright=2.5|350px|alt=Sun spectral irradiance measured with a calibrated spectrometer (Flame S-XR1-ES, Ocean Insight) mounted with a cosine corrector. The measurement was made on a sunny day on 13&nbsp;June 2022 at noon from Hauterive, Switzerland (47°01′N &nbsp; 6°58′E). For photometry and colorimetry, standard measurements are usually carried out in the range 360–830&nbsp;[[nannometer|nm]] (highlighted area). Correlated color temperature (CCT): 5470&nbsp;[[Kelvin (unit)|K]]. | Solar spectral irradiance measured with a calibrated [[optical spectrometer]] mounted with a cosine corrector. Some of the characteristic Fraunhofer lines and their corresponding elements are indicated for the extended [[visible spectrum]] (highlighted area in the graph). <br>
For photometry and colorimetry, standard measurements are usually carried out in the range 360–830 nm. From these data and for this spectral range, the [[correlated color temperature]] (CCT) is 5470 K.]]
{| <!-- Start nested table -->
| valign="top" |
{| class="wikitable"  style="text-align:center;"
|- style="vertical-align:bottom;"
! Line !! Element !! Wavelength<br/>([[nanometer|nm]])
|- 
| y || [[Oxygen|O{{sub|2}}]] || 898.765
|-
| Z || O{{sub|2}} || 822.696
|-
| A || O{{sub|2}} || 759.370
|-
| B || O{{sub|2}} || 686.719
|-
| C || [[Hα|H{{sub|{{math|α}} }}]] || 656.281
|-
| a || O{{sub|2}} || 627.661
|-
| D{{sub|1}} || [[Sodium|Na]] || 589.592
|-
| D{{sub|2}} || Na || 588.995
|-
| D{{sub|3}} (or d) || [[Helium|He]] || 587.5618
|-
| e || [[Mercury (element)|Hg]] || 546.073
|-
| E{{sub|2}} || [[Iron|Fe]] || 527.039
|-
| b{{sub|1}} || [[Magnesium|Mg]] || 518.362
|-
| b{{sub|2}} || Mg || 517.270
|-
| b{{sub|3}} || Fe || 516.891
|-
| b{{sub|4}} || Mg || 516.733
|}
| valign="top" | <!-- Start second half of the nested table -->
{| class="wikitable"  style="text-align:center;"
|- style="vertical-align:bottom;"
! Line !! Element !! Wavelength<br/>([[nanometer|nm]])
|- 
| c  || Fe || 495.761
|-
| F  || [[Hβ|H{{sub|{{math|β}} }}]] || 486.134
|-
| d  || Fe || 466.814
|-
| e  || Fe || 438.355
|-
| G′ || [[Hγ|H{{sub|{{math|γ}} }}]] || 434.047
|-
| G  || Fe || 430.790
|-
| G  || [[Calcium|Ca]] || 430.774
|-
| h  || [[Hδ|H{{sub|{{math|δ}} }}]] || 410.175
|-
| H  || Ca{{sup|+}} || 396.847
|-
| K  || Ca{{sup|+}} || 393.366
|-
| L  || Fe || 382.044
|-
| N  || Fe || 358.121
|-
| P  || [[Titanium|Ti]]{{sup|+}} || 336.112
|-
| T  || Fe || 302.108
|-
| t  || [[Nickel|Ni]] || 299.444
|}
|} <!-- End of nested table -->

[[File:cmglee_Cambridge_Science_Festival_2016_sodium_lines.jpg|thumb|300px|A demonstration of the 589&nbsp;nm D<sub>2</sub> (left) and 590&nbsp;nm D<sub>1</sub> (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 the [[Balmer series]] of [[emission line]]s of the hydrogen atom. The Fraunhofer letters are now rarely used for those lines.

The D<sub>1</sub> and D<sub>2</sub> lines form a pair known as the "sodium doublet", the centre wavelength of which (589.29&nbsp;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 D<sub>1</sub> and D<sub>2</sub> lines correspond to the [[fine structure|fine-structure]] splitting of the excited states.

The Fraunhofer H and K&nbsp;letters are also still used for the calcium{{sup|II}} 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&nbsp;line may refer to the [[cyan]] iron line at 466.814&nbsp;nm, or alternatively to the [[yellow]] helium line (also labeled D<sub>3</sub>) at 587.5618&nbsp;nm. Similarly, there is ambiguity regarding the e&nbsp;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&nbsp;line and Helium d&nbsp;line).

Because of their well-defined wavelengths, Fraunhofer lines are often used to specify standard wavelengths for characterising the [[refractive index]] and [[dispersion (optics)|dispersion]] properties of optical materials.

==See also==
*[[Abbe number]], measure of glass dispersion defined using Fraunhofer lines
*[[Timeline of solar astronomy]]
*[[Spectroscopy|Spectrum analysis]]

==References==
{{Reflist|30em}}

==Further reading==
*{{cite book |author1=Myles W. Jackson |author2=Albert Gallatin Research Excellence Professor of the History of Science at Nyu-Gallatin and Professor Myles W Jackson |title=Spectrum of Belief: Joseph Von Fraunhofer and the Craft of Precision Optics |url=https://books.google.com/books?id=VJ6SE3sbxDsC |year=2000 |publisher=MIT Press |isbn=978-0-262-10084-7}}

==External links==
{{Commons|Fraunhofer lines}}
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