[ spectral nature of light | atomic identity of line spectra ]

The Era of Classical Spectroscopy

Spectral nature of light

Although the spectral nature of light is present in the rainbow, it was beyond the ability of early man to recognize its significance. It was not until 1666 that Newton showed that the white light from the sun could be dispersed into a continuous series of colors. Newton introduced the word "spectrum" to describe this phenomenon. His instrument employed a small aperture to define a beam of light, a lens to collimate it, a glass prism to disperse it, and a screen to display the resulting spectrum. This first spectroscope was nearly in modern form. Newton' s analysis of light was the beginning of the science of spectroscopy.

It gradually became clear that the sun's radiation has components outside the visible portion of the spectrum. W Herschel (1800) demonstrated that the sun's radiation extended into the infrared, and J.W. Ritter (1801) made similar observations in the ultraviolet. These studies were the precursors of radiometric and photographic measurements of light, respectively.
Spectral lines and their quantitative measurement

The achievements of Joseph Fraunhofer provided the quantitative basis for spectroscopy. Fraunhofer, born near Munich in 1787, extended Newton's discovery by observing that the sun's spectrum, when sufficiently dispersed, was crossed by a large number of fine dark lines (1814), now known as Fraunhofer lines. W.H. Wollaston had earlier observed a few of these lines (1802), but failed to attach any significance to them. These were the first spectral lines ever observed, and Fraunhofer employed the most prominent of them as the first standards for comparing spectral lines obtained using prisms of different glasses. Fraunhofer also studied spectra of the stars and planets, using a telescope objective to collect the light. This laid the foundation for the science of astrophysics.

Fraunhofer also developed the diffraction grating, an array of slits, which disperses light in much the same way, as does a glass prism, but with important advantages. With a prism, the angle at which a spectral line is dispersed depends on the type of glass used. This makes it difficult to compare different spectral measurements, and absolute wavelength measurement is not possible. Gratings, which employ interference of light waves to produce diffraction, provide a means of directly measuring the wavelength of the diffracted beam. Earlier, T. Young had demonstrated that a light beam passing through a slit emerges in a pattern of bright and dark fringes. Fraunhofer extended these studies to the case of two, three and many closely spaced slits, and thus developed the transmission grating. With this, he was able to directly measure the wavelengths of spectral lines. Fraunhofer's achievements are all the more impressive, considering that he died at the early age of 39.

Atomic identity of line spectra

Despite his enormous achievements, Fraunhofer did not understand the origin of the spectral lines he observed. It was not until 33 years after his death that Kirchhoff established that each element and compound has its own unique spectrum, and that by studying the spectrum of an unknown source, one can determine its chemical composition. With these advancements, spectroscopy became a true scientific discipline.
In the early 1800's many workers, J.F.W. Herschel, W.H.F. Talbot, C. Wheatstone, A.J. Angstrom, and D. Alter among them, studied spectra from terrestrial sources such as flames, arcs and sparks. These sources were found to emit bright spectral lines, which were characteristic of the chemical elements in the flame. Foucault, the French physicist, observed in 1848 that a flame containing sodium would absorb the yellow light emitted by a strong arc placed behind it. This was the first demonstration of a laboratory absorption spectrum.

These facts were brought together in 1859 by G. Kirchhoff in his famous law, which states that the emitted power and absorbed power of light at a given wavelength are the same for all bodies at the same temperature. It follows that a gas, which radiates a line spectrum must, at the same temperature, absorb the spectral lines it radiates. From this, Kirchhoff and R. Bunsen explained that the Fraunhofer lines in the sun' s spectrum were due to absorption of the continuous spectrum emitted from the hot interior of the sun by elements at the cooler surface. Analysis of the sun's atmosphere thus became possible.

By recognizing that each atom and molecule has its own characteristic spectrum, Kirchhoff and Bunsen established spectroscopy as a scientific tool for probing atomic and molecular structure, and founded the field of spectrochemical analysis for analyzing the composition of materials. These techniques are used today to analyze both terrestrial and stellar objects, and it continues to be our only means of studying the chemical elements present in stars.