Optical research in Leiden
Zeeman studied in Leiden with Kamerlingh Onnes and Lorentz and received his doctorate in 1893. His early optical investigations included the Kerr effect. He moved to Amsterdam in 1897 and later directed its physics laboratory.
References: [1]
Spectral lines in magnetic fields
In 1896 Zeeman found that a spectral line could split into several components in a magnetic field. Lorentz’s electron theory explained some cases. Other observations, called the anomalous Zeeman effect, required later developments in quantum theory.
From laboratory spectra to the Sun
Zeeman recognised that changes in solar spectral lines could reveal magnetic fields on the Sun. George Ellery Hale’s observations in 1908 supported this application. Spectral splitting and polarisation thus became tools for investigating astronomical environments as well as laboratory matter.
References: [1]