What is Spectroscopy?

By BSc (Hons)

Spectroscopy is a technique that utilizes the interaction of different frequency components of the electromagnetic spectrum with a sample to perform an analysis.

Historically, spectroscopy referred to the use of visible light dispersed according to its wavelength by a prism. This was done by Isaac Newton as part of his Optics experiments in 1666 when he constructed an instrument to induce dispersion of a beam of light into a continuous spectrum of colors. This first analysis of light was the start of the science of spectroscopy.

Further experimentation made it clear that the sun's radiation includes more than just the visible region of the spectrum that Newton had explored. W Herschel (1800) discovered the infrared region by studying the heating ability of the colors of the spectrum. The following year, J.W. Ritter similarly determined the existence of the ultraviolet region by testing the effect of the colors of the spectrum on silver chloride.

Instead of the prism which Newton had used, Joseph von Fraunhofer made use of a diffraction grating to induce light (wavelength) dispersion. He made this grating by closely spacing thousands of slits so that the interference he achieved was improved in both the spectral resolution and allowed quantification of the dispersed wavelengths.

He then utilized the previously established theories of light interference (Thomas Young, François Arago and Augustin-Jean Fresnel) as the centre of his own experiments to demonstrate the effect of passing light through rectangular slits.

In the 1820s both John Herschel and William H. F. Talbot used flame spectroscopy for the spectral analysis of elements. This technique allowed them to demonstrate that when a substance is heated and its light is passed through a spectroscope, each element emits its own characteristic bright lines of colors.

There were a great many postulates that arose following this but it was in the 1860s that spectroscopy experienced a period of great advancements due to its applications in the analysis of compounds. The physicist Gustav Robert Kirchhoff, the chemist Robert Wilhelm Bunsen and the optician Carl August von Steinheil, who manufactured a spectroscope in his workshop, lay the foundation for the scientific and technological applications of spectroscopy through their published “Chemical Analysis through Spectral Observations.”

This was centred on the linkage between chemical elements and their unique spectral patterns. They demonstrated that every metal, whatever compound it was in, yielded the same spectrum. With this fact, they used spectral testing for numerous trace chemical analyses and discovered the previously unknown elements, cesium and rubidium. They even used their analysis to observe the Sun and determine that it was mostly composed of Hydrogen gas.

In 1861, William and Margaret Huggins detected the same elements in stars as had already been detected in the Sun by Bunsen and Kirchoff — there was no doubt to the claim that the stars are Suns, and therefore, the Sun must also be a star too. This conclusion was reached when the pair combined the telescope to capture the light of a star with the spectroscope to disperse the starlight into its constituent colors.

In 1868, when observing the brightest star in the sky (Sirius) they found that the spectral lines were slightly shifted in the direction of the red side of the spectrum. This shift in the spectral lines indicated that Sirius was traveling away from Earth at approximately 135 miles/second.

In a similar way when they measured other stars, they found that a few exhibited a shift in the direction of the blue side of the spectrum, indicating the opposite i.e that they were traveling towards Earth. This method (named the Doppler shift), would ultimately be useful in studying faint clouds (nebulae) and would eventually lead to the established discovery of the expanding universe in the 20th century.



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Last Updated: Nov 23, 2015

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  1. Gogoylini Mp Gogoylini Mp Greece says:

    I find your article very intresting. Although i would like to make a question: what do you mean when you say that spectroscopy can refer to a response to an alternating field or varying frequency ? I am expecting your reply anxiously,

    Τhank you in advance

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