A long tradition of studying the effect of magnetic fields on light emitted by atoms dates back to Michael Faraday. Today, the inevitability of the existence of the effects of such an influence seems to us obvious, since we know that electrons and other atoms have spin, that is, they behave like microscopic electrically charged tops that form a magnetic field around them, and, in fact, are microscopic magnets (cm. Stern-Gerlach experiment). At the end of the 19th century, when Peter Zeeman decided to conduct a series of experiments and check whether atoms have magnetic properties, everything was, however, far from so obvious. The scientist placed a tiny sample of sodium between the poles of an adjustable magnet and began to study the effect of a magnetic field on the spectral emission lines of sodium atoms (cm. Spectroscopy). It turned out that with an increase in the magnetic field, the spectral lines in each frequency group are blurred, that is, new radiation frequencies appear in them. This was the first unambiguous confirmation of the existence of the effect, which will later be called the Zeeman effect.
To understand its nature, the easiest way is to turn to Bohr’s model of the atom and think about how exactly the light is emitted. An electron makes a quantum leap from a higher orbit to a lower one (or, which is the same, from a higher energy level to a lower one), while emitting a photon of a strictly defined frequency corresponding to the energy difference between the two energy levels. Now, if we assume that the electron is actually a microscopic magnet, and the atom itself is placed in an external magnetic field, the energy of the electron will depend on the polarity of its magnetic spin – if the electron’s magnetic field in orbit is unidirectional to the external magnetic field, it has one energy, if however, it is oriented in the opposite direction, then the other. That is, electrons with opposite magnetic spin, located in the same orbital, will have slightly different energies, and each energy level will be split into two close sublevels. Accordingly, where there used to be a single possible quantum transition energy between two levels, there are now four possible transition energies. This should be reflected in the radiation spectrum in such a way that instead of one clearly distinguished spectral line (radiation frequency), four closely spaced equidistant spectral lines (frequencies) appear in a powerful magnetic field.
In the initial experiment, Zeeman was unable to distinguish between these four spectral lines, since the imperfection of the spectroscope and the insufficient power of the magnet led to the fact that instead of splitting, a simple smearing of the spectral lines was observed. However, later the scientist managed to improve the equipment and identify four separate spectral lines in place of one blurry one, as the theory predicted. This required an increase in the magnetic field, and Zeeman even managed to prove that the distance between the split lines of the spectrum directly depends on the strength of the magnetic field.
The Zeeman effect later found a very useful application in astronomy, since the splitting of lines in the radiation spectrum of celestial bodies can be used to judge the strength of their magnetic fields. For example, it was by the Zeeman effect that astrophysicists were able to establish that sunspots on the Sun are a consequence of the disturbance of powerful magnetic fields near its surface – solar magnetic storms.