A Short History of Light: The Spectroscope
By Stephen W. and Nancy L. Attaway
The most familiar spectrum in nature is that splendid spectacle, the rainbow, produced when light from the sun bounces around inside each of millions of raindrops and is sorted out into its constituent colors in the process. In 1666, Newton discovered that sunlight passed through a prism reveals the colors of the rainbow. The arrangement of the colors of the rainbow is called the color spectrum. A very close look at sunlight passed through a prism reveals that the color spectrum is not continuous, but is actually composed of discrete lines of color.
Pulitzer Prize winner, Richard Rhodes wrote this historical account on the connection between the color spectrum and the advancing field of atomic physics: “Spectroscopy was a well-developed field in 1912. The eighteenth-century Scottish physicist Thomas Melvill had first productively explored it. He mixed chemical salts with alcohol, lit the mixtures, and studied the resulting light through a prism. Each different chemical produced characteristic patches of color, which suggested the possibility of using spectra for chemical analysis to identify unknown substances. The prism spectroscope, invented in 1859, advanced the science. The spectroscope used a narrow slit set in front of a prism to limit the patches of light to similarly narrow lines. These could be directed onto a ruled scale (and later onto strips of photographic film) to measure their spacing and calculate their wavelengths. Such characteristic patterns of lines came to be called line spectra. Every element had its own unique line spectrum. Helium was discovered in the chromosphere of the sun in 1868 as a series of unusual spectral lines twenty-three years before it was discovered mixed into uranium ore on earth. The line spectra had their uses.” “But no one understood what produced the spectrum lines. At best, mathematicians and spectroscopists who liked to play with wavelength numbers were able to find beautiful harmonic regularities among sets of spectral lines.”
In 1913, Niels Bohr was studying the orbit of electrons around an atom. He was the first to connect the spectrum lines to the orbits of electrons. Bohr established the relationship between the orbiting electrons and the lines of spectral light. “Bohr proposed that an electron bound to a nucleus normally occupies a stable, basic orbit called a ground state. Add energy to the atom, heat it, for example, and the electron responds by jumping to a higher orbit, one of the more energetic stationary states farther away from the nucleus. Add more energy, and the electron continues jumping to higher orbits. Cease adding energy, leaving the atom alone, and the electrons jump back to their ground states. With each jump, each electron emits a photon of characteristic energy.”
Thus, light is made up of photons of energy. Each photon has a characteristic energy associated with the transition of an electron moving from one state to another. The color of light is related to the amount of energy needed to move from one state to another. Light shone on certain metals knocks electrons free. However, the energy of the electrons knocked free of the metal does not depend upon the brightness of light, but, instead, hinges upon the color of light, on its frequency.
As light passes through a gem, the specific wavelengths of light can
be absorbed. If you take light that has been passed through a gem and use
a prism or a diffraction grading spectroscope to spread the light into
a wide band, these absorbed wavelengths manifest as lines or areas of darkness
in the spectrum. It is possible to measure the actual wavelengths that
are absorbed and use them for identification. The most common approach
to gemstone identification is to use the pattern of lines. Matching the
absorption spectrum is the fastest way to determine the chemical composition
of large or small numbers of stones. The red gems, like spinel, ruby, and
tourmaline all have distinctive spectra. There are well defined absorption
spectrums for synthetic verneuil sapphire, for blue synthetic spinel, and
for almandine garnet, just to name a few. In real life, the absorption
spectrum lines you see from a gemstone are really faint, fuzzy, and hard
to see. The most realistic drawings available are found in the Handbook
of Gem Identification by Richard T. Liddicoat, Jr.