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  the Fraunhofer Spectrum
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the Fraunhofer spectrum


In 1802, a scientist called W.H. Wollaston noticed that the visible spectrum from the Sun had several dark lines in it. Not long afterwards, Joseph von Fraunhofer built the first spectrometer. This focused sunlight from a small telescope onto a narrow slit. The light then passed through a prism, which produced the spectrum. Fraunhofer later invented the diffraction grating, which is used in most spectrometers today.



Joseph von Fraunhofer


Fraunhofer not only confirmed Wollaston's results, but also found that there were far more dark lines in the spectrum than Wollaston had suspected. Fraunhofer showed that these were a feature of sunlight and not an illusion nor an optical effect, and he labelled them with letters of the alphabet (A,B,C etc.). We now call these dark lines Fraunhofer lines.

What are the Fraunhofer lines and how are they formed?
When the visible light from below the Sun's surface passes through the layers above it (the photosphere and chromosphere), some of the light at particular wavelengths is absorbed by atoms and ions and so is missing in the spectrum we see. When there is no light it appears as black in the spectrum.


Here is a spectrum showing Fraunhofer lines


The dark lines in the spectrum occur when light is absorbed at a particular wavelength.

mouse over arrow K and H are two lines from calcium at 396.8 and 393.4 nm


mouse over arrow D corresponds to two lines from sodium at 589.0 and 589.6 nm

mouse over arrow C is a line from hydrogen at 656.3 nm


When light passes through material and is absorbed by atoms and ions of each element (hydrogen, calcium, sodium, iron etc.) a unique set of dark lines is formed in the spectrum. These are called absorption lines.


Recognise this?
Look on any pack of food from your local supermarket. The Bar Code is scanned at the checkout. It has all the information needed to identify the product. Each identical product has the same bar code. In just the same way, the atoms or ions of each element have a unique spectrum.

Since the composition of every star, including the Sun, is unique, the pattern of spectral lines in its spectrum provides us with a fingerprint for the star.





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