Spectroscopy

In the year 1666, Isaac Newton held a prism up to an aperture through which light from our nearest star shone through. In doing so, Newton dispersed the star light into a band of colours from red to violet in what is called the solar spectrum, projecting it onto a nearby screen, before cohering the spectrum of colours back into white light by means of a lens and a second prism. However, Newton remained unaware as to what vital information actually lay hidden in the spectra he had just produced before him.

Newton disperses sunlight into it's spectrum.

Newton disperses sunlight into it’s spectrum.

A French philosopher called Auguste Comte, in the year 1835 published a book entitled ‘Positive Philosphy’. In this book he wrote of the stars, “We see how we may determine their forms, their distances, their bulk, their motions but we can never know anything of their chemical or mineralogical structure”. Yet, 33 years earlier, in 1802, William Hyde Wollaston, directed sun light through a prism and observed fine dark spectral lines superimposed on the spectrum where the light had been absorbed at discreet wavelengths. These dark lines were then studied in detail by a poor glass-polisher’s apprentice named Josef von Fraunhofer, who in 1814 had catalogued 475 of these dark lines in the solar spectrum. While Franhofer precisely measured the wavelengths at which these lines occurred along the spectrum, he made a startling discovery! In the yellow region of the spectrum, he realised that a wavelength measurement he had just made, corresponded with the wavelength of the yellow light emitted when grains of sodium chloride, commonly known as table salt, are sprinkled in a flame, proving Auguste Comte wrong. And in doing so, he discovered that these dark lines were in fact spectral absorption lines. With the identification of this sodium line, the science of spectroscopy was born.

In memory of Fraunhofer.

In memory of Fraunhofer.

Thirty four years later, in 1849, Leon Foucault studied the emission spectra of Sodium. Ten years later, the researchers, Gustav Kirchoff and Robert Bunsen identified the sodium emission lines buried in the solar spectrum. This marked a major advancement in spectroscopy. They then realised that each and every element in the periodic table of elements bears it’s own unique spectral finger print that is truly unique to that element and is insolubly keyed to that element as the lines of your finger prints are keyed to you. Kirchoff formalized three laws of spectroscopy to describe the spectral compsosition of light emitted by incandescent objects; 1] a solid object, whose internal energy is raised to a sufficient level, emits a continuous spectrum of light, 2] a tenuous gas, whose temperature is raised to a sufficient level, emits spectral lines that occur at discreet wavelengths, depending on what elements the gas is composed of, and 3] a solid object with a sufficient amount of internal energy surrounded by a cool tenuous gas emits light with an almost continuous spectrum which has gaps at discrete wavelengths depending on the energy levels of the atoms in the gas.

Appearing very similar to a bar code, each dark line in a spectrum indicates a wavelength absorbed whilst each bright line indicates an emission wavelength. Light passing through a relatively cool gas, such as the temperature minimum region of the Sun for example, produces an absorption spectrum, in this case called the Fraunhofer spectrum. Kirchhoff did not know about the existence of energy levels in atoms. The existence of discrete spectral lines was later explained by the Bohr model of the atom, which helped lead to quantum mechanics.

Angelo Secchi of the Vatican Observatory began recording and analyzing the spectra of stars, accumulating over 4,000 spectrograms. He discovered that stars could be classified into a number of distinct types and subtypes, which could be distinguished by their spectral types (ie. by the number and strength of absorption lines in their spectra). Three groups emerged. Blue and White stars, yellow stars, orange and red stars. It was later found that the spectral types of the stars were related to their surface temperatures. Particular absorption lines can be observed through a certain temperature range because it is only in this temperature range that the atomic energy levels involved can be populated. In the late 1890’s, the Secchi classification scheme began to be replaced by the Harvard spectral classification scheme.

In 1868, Sir Norman Lockyer observed a peculiar set of yellow emission lines in the solar spectrum which he concluded must be due to some other unknown element, which he named Helium from the Greek word helios meaning Sun.

When the British astronomer William Huggins peered through the telescope at the Cat’s Eye nebula, he was astonished to find isolated emission lines. It was later discovered in the 1920s that the emission lines were due to ionised oxygen atoms. Nebulae are extremely rarefied, to a much higher degree than the hardest vacuum ever produced here on Earth. In these conditions, atoms behave quite differently and lines can form which are suppressed at normal densities. These lines are known as forbidden lines, and are the most intense lines in most nebular spectra.

In 1913 Niels Bohr established a model for the atom now known as the Bohr model. His model described the atom comprising of a central nucleus surrounded by electrons in spherical orbits of different energies. At the same time he also derived a relationship that defined the relationship between the spectral emission / absorption wavelength and the energy level in his atomic model. Using this relationship Bohr, was able to calculate the energy levels of the hydrogen atom and thus the atomic emission / absorption lines, in agreement with experimental results.

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