What Are Fraunhofer Lines in Physics?

The Fraunhofer lines are a set of famous absorption lines named after German physicist Joseph von Fraunhofer. Fraunhofer identified and designated the principal features with the letters A through K from a longer wavelength (red region of the spectrum) to a shorter (blue region of the spectrum). When a solar spectrum is closely examined, it is found that it consists of several dark lines. These dark lines found in the solar spectrum are known as the Fraunhofer lines. The solar spectrum is found to be an example of a line absorption spectrum. 

The central core of the sun which is known as the photosphere is at a very high temperature of order 14 million kelvin and this part of the sun emits a continuous spectrum. The outer layer of the sun called the chromosphere is comparatively at a lower temperature about 6000 Kelvin it contains various elements in the gaseous state and it is the reason for the solar spectrum and the formation of Fraunhofer lines.

Fraunhofer Lines of Sun: 

When light from the central core (i.e., photosphere) of the sun passes through the sun’s atmosphere, certain wavelengths are absorbed by the elements presents in the chromosphere (i.e., the outermost layer of the sun), this will result in the formation of dark lines in the solar spectrum, and theses dark lines that are present in the solar spectrum are known as the Fraunhofer lines of the sun or just the Fraunhofer lines. The spectrum with the Fraunhofer lines of the sun also called the Fraunhofer spectrum. 

The German physicist Joseph von Fraunhofer (1787–1826) invented a device called a spectroscope that contained a diffraction grating. When Fraunhofer used this device to analyse the light emitted from the Sun he observed dark lines on a continuous spectrum and they are known as the Fraunhofer lines.

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By comparing the absorption spectra of various elements with the Fraunhofer lines in solar spectrum, various elements present in the sun’s atmosphere have been identified. Many characteristics properties of the sun can be found with the help of the solar spectrum and particularly to study the elements present in the sun the Fraunhofer lines are found to be important.

Fraunhofer Spectrum:

In 1814, German physicist Joseph von Fraunhofer studied and measured the dark lines that are present in the solar spectrum. 45 years later, it was noticed and observed that the lines coincide with the emission lines in the spectra of heated elements. The discovery allows us to determine the composition of the Sun and the elements present in the sun. The Fraunhofer spectrum is shown in the figure below:

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The Fraunhofer lines were first observed in the year 1802 by the English physicist William Hyde Wollaston, but they are named after the German physicist Joseph von Fraunhofer, who in 1814 plotted more than 500 Fraunhofer lines and designated the brightest lines by the letters A through G, and this system of identification is still in use. For example, the D line in the Fraunhofer spectrum is caused by sodium, and the H and K lines are caused by calcium. Some Fraunhofer absorption lines were known to originate due to absorption in the Earth's atmosphere. About 25,000 Fraunhofer lines are now identified to exist in the solar spectrum, between the wavelengths of 2,950 and 10,000 angstroms. (We know that one angstrom equals 10-8 cm)

We now know that the Fraunhofer lines are absorption lines. But how are these lines produced, and what do they show us? The Sun and the other stars produce all wavelengths of light. As this light passes through the cooler outer atmosphere, gas atoms absorb certain wavelengths of light, producing a line absorption spectrum that we see from Earth. Scientists in the 19th century were able to examine and compare these dark lines with the line emission spectra of the known elements and identify what elements were in the cooler atmosphere.

The Fraunhofer absorption lines are, indeed, a boon and lifeline of solar physicists. The deep insight of the absorption lines provide information regarding temperature, and whereas the wavelength shifts of the lines provide information about the motion of the gas. If the Sun consisted only of elements such as pure hydrogen, there would be no existence of an absorption line. This would mean that the researchers would not have been able to study the temperature or the motion of the Sun's atmosphere. This would be the end for them. Thanks to the impurities and the elements, we can investigate the Sun in detail.

Did You Know:

• The German physicist Joseph Fraunhofer identified around 700 lines in the solar spectrum in the early 1800s, now we know that there are many thousands of lines in the Sun's spectrum, caused by different chemical elements in the solar atmosphere.

• About 50% of the energy is in the visible wavelengths below 0.7 μm. We can tell this by doing a quick integration.

• O₃ and O₂ absorb much of the UV irradiance below 300 nm high in the atmosphere.

• About 70% of the visible irradiance makes it all the way to sea level.

• O₃ absorbs a little of the visible irradiance.

• A significant fraction of the visible irradiance is scattered by clouds and aerosol. Some are reflected out into space so that this portion never deposits energy in the Earth system.

• There are large wavelength bands in which water vapour, carbon dioxide, and O3absorb infrared irradiance.