The term "doublet" dates back to the early 19th century, when it was observed that certain
spectral lines of an ionized, excited gas would split into two under the influence of a strong
magnetic field, in an effect known as the
anomalous Zeeman effect. Such spectral lines were observed not only in the laboratory, but also in astronomical
spectroscopy observations, allowing astronomers to deduce the existence of, and measure the strength of magnetic fields around the Sun, stars and galaxies. Conversely, it was the observation of doublets in spectroscopy that allowed physicists to deduce that the
electron had a spin, and that furthermore, the magnitude of the spin had to be . See the history section of the article on
spin (physics) for greater detail. Doublets continue to play an important role in physics. For example, the
healthcare technology of
magnetic resonance imaging is based on
nuclear magnetic resonance. In this technology, a spectroscopic doublet occurs in a
atomic nucleus, whose doublet splitting is in the radio-frequency range. By applying both a magnetic field and carefully tuning a radio-frequency transmitter, the nuclear spins will flip and re-emit radiation, in an effect known as the
Rabi cycle. The strength and frequency of the emitted radio waves allow the concentration of such nuclei to be measured. ==See also==