Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Fraunhofer lines

Solar spectrum with Fraunhofer lines as it appears visually.

Spectrum of a blue sky somewhat close to the horizon pointing east at around 3 or 4 pm on a clear day.

In physics and optics, the Fraunhofer lines are a set of spectral lines named for the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.

The English chemist William Hyde Wollaston was in 1802 the first person to note the appearance of a number of dark features in the solar spectrum. In 1814, Fraunhofer independently rediscovered the lines and began a systematic study and careful measurement of the wavelength of these features. In all, he mapped over 570 lines, and designated the principal features with the letters A through K, and weaker lines with other letters. Modern observations of sunlight can detect many thousands of lines.

It was later discovered by Kirchhoff and Bunsen that each chemical element was associated with a set of spectral lines, and deduced that the dark lines in the solar spectrum were caused by absorption by those elements in the upper layers of the Sun. Some of the observed features are also caused by absorption in oxygen molecules in the Earth's atmosphere.

The major Fraunhofer lines, and the elements they are associated with, are shown in the following table:

Designation	Element	Wavelength (nm)	Designation	Element	Wavelength (nm)
y	O2	898.765	c	Fe	495.761
Z	O2	822.696	F	Hβ	486.134
A	O2	759.370	d	Fe	466.814
B	O2	686.719	e	Fe	438.355
C	Hα	656.281	G'	Hγ	434.047
a	O2	627.661	G	Fe	430.790
D1	Na	589.592	G	Ca	430.774
D2	Na	588.995	h	Hδ	410.175
D3 or d	He	587.5618	H	Ca+	396.847
e	Hg	546.073	K	Ca+	393.368
E2	Fe	527.039	L	Fe	382.044
b1	Mg	518.362	N	Fe	358.121
b2	Mg	517.270	P	Ti+	336.112
b3	Fe	516.891	T	Fe	302.108
b4	Fe	516.891	t	Ni	299.444
b4	Mg	516.733

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 D₁ and D₂ lines form the well-known "sodium doublet", the center wavelength of which (589.29 nm) is given the designation letter "D". This historical designation for this line has stuck and is given to the 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 splitting of the excited stated. This may be confusing because the excited state for this transition is the P-state of the alkali and should not be confused with the higher D-states.

Note that there is disagreement in the literature for some line designations; e.g., 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 with reference to 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 characterize the refractive index and dispersion properties of optical materials.