Atomic Light Signatures
Year of Discovery: 1859
What Is It? When heated, every element radiates light at very specific and
characteristic frequencies.
Who Discovered It? Gustav Kirchhoff and Robert Bunsen
Why Is This One of the 100 Greatest?
Twenty new elements (beginning with the discovery of cesium in 1860) were discovered
using one chemical analysis technique. That same technique allows astronomers to determine
the chemical composition of stars millions of light years away. It also allowed physicists to
understand our sun’s atomic fires that produce heat and light. That same technique allows
other astronomers to calculate the exact speed and motion of distant stars and galaxies.
That one technique is spectrographic analysis, the discovery of Kirchhoff and Bunsen,
which analyzes the light emitted from burning chemicals or from a distant star. They discovered that each element emits light only at its own specific frequencies. Spectrography provided the first proof that the elements of Earth are also found in other heavenly bodies—that
Earth was not chemically unique in the universe. Their techniques are routinely used by scientists in virtually every field of science in the biological, physical, and earth sciences.
How Was It Discovered?
In 1814, German astronomer Joseph Fraunhofer discovered that the sun’s energy was
not radiated evenly in all frequencies of the light spectrum, but rather was concentrated in
spikes of energy at certain specific frequencies. Some thought it interesting, none thought it
important. The idea lay dormant for 40 years.
Gustav Kirchhoff (born in 1824) was an energetic Polish physicist who barely stood
five feet in height. Through the mid-1850s he focused his research on electrical currents at
the University of Breslau. In 1858, while helping another professor with a side project,
Kirchhoff noted bright lines in the light spectrum produced by flames and recalled having
read about a similar occurrence in Fraunhofer’s articles. Upon investigation, Kirchhoff
found that the bright spots (or spikes) in the light from his flame studies were at the exact
same frequency and wave lengths that Fraunhofer had detected in solar radiation.
Kirchhoff pondered what this could mean and was struck by what turned out to be a
brilliant insight: use a prism to separate any light beam he wanted to study into its constituent parts (instead of peering at it through a sequence of colored glass filters as was the cus-
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