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What are Fraunhofer lines?

The Fraunhofer lines are any of the dark absorption lines in the spectrum of stars (like the Sun), which is caused by selective absorption of a star’s radiation at specific wavelengths by various gas elements existing in the atmosphere.

First observed by an English physicist William Hyde Wollaston in 1802 but are named after a German physicist Joseph von Fraunhofer. Since 1814 Fraunhofer plotted more than 500 Fraunhofer lines and assigned the brightest by letters A to G, which is still in use today.

At the time of writing this, there are 25,000 Fraunhofer lines which are known to exist in our Solar System spectrum.

There are three kinds of spectra you should know about.

Continuous – All the colours, i.e. wavelengths, of the visible spectrum with nothing missing. What we see when white light is dispersed by a prism.

continous spectrum
Image via: NASA

 

 

 

Emission – When atoms are excited they give off light. Different elements produce light with very definite wavelengths, i.e. very definite colours. Sodium lamps are yellow because sodium emits lots of yellow light. Neon lights are red because neon atoms emit lots of red and orange light. Mercury emits lots of blue and green light. Think of it as a fingerprint or a barcode. If we can measure the wavelength of the light given off by a substance, we can identify it. Here is the visible part of the emission spectrum for hydrogen.

spectrum emission
Image via: NASA

Absorption – If white light, containing all the colours, passes through a gas then when it emerges some of the colours are missing. This is because the atoms in the gas absorb light with very definite wavelengths, actually exactly the same wavelengths that the same atoms would emit if they were excited. What we see is a continuous spectrum with black lines on it. Here is an absorption spectrum for hydrogen. Notice what colours are missing.

spectrum absorption
Image via: NASA

If we pass sunlight through a prism, we see the visible spectrum we expect but also many black lines. It is called the Fraunhofer spectrum after one of the first people to study it. Below is a German stamp featuring Fraunhofer’s original drawing.

This is, of course, an absorption spectrum. The black lines, over 600 of them, are due to the absorption of particular wavelengths of light by chemical elements in the outer layers of the Sun. This means that if we measure the wavelength of these black lines, we can identify what elements there actually are in the Sun. Their relative intensity also tells us how much of these elements are present and so we can build up a pretty good picture of what the Sun is made of.

One set of lines, including a strong yellow line, did not correspond to any known element. In 1870 Lockyer suggested that they corresponded to a new element which he named Helium after the Greek Sun god Helios. 25 years later Helium was discovered on Earth.

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