text: After Zeeman passed the qualification exams in 1885, he studied physics at the University of Leiden under Kamerlingh Onnes and Hendrik Lorentz. In 1890, even before finishing his thesis, he became Lorentzs assistant. This allowed him to participate in a research programme on the Kerr effect. In 1893 he submitted his doctoral thesis on the Kerr effect, the reflection of polarized light on a magnetized surface. After obtaining his doctorate he went for half a year to Friedrich Kohlrauschs institute in Strasbourg. In 1895, after returning from Strasbourg, Zeeman became Privatdozent in mathematics and physics in Leiden. The same year he married Johanna Elisabeth Lebret (1873–1962); they had three daughters and one son.
  In 1896, shortly before moving from Leiden to Amsterdam, he measured the splitting of spectral lines by a strong magnetic field, a discovery now known as the Zeeman effect, for which he won the 1902 Nobel Prize in Physics. This research involved an investigation of the effect of magnetic fields on a light source. He discovered that a spectral line is split into several components in the presence of a magnetic field. Lorentz first heard about Zeemans observations on Saturday 31 October 1896 at the meeting of the Royal Netherlands Academy of Arts and Sciences in Amsterdam, where these results were communicated by Kamerlingh Onnes. The next Monday, Lorentz called Zeeman into his office and presented him with an explanation of his observations, based on Lorentzs theory of electromagnetic radiation.
  The importance of Zeemans discovery soon became apparent. It confirmed Lorentzs prediction about the polarization of light emitted in the presence of a magnetic field. Thanks to Zeemans work it became clear that the oscillating particles that according to Lorentz were the source of light emission were negatively charged, and were a thousandfold lighter than the hydrogen atom. This conclusion was reached well before J. J. Thomsons discovery of the electron. The Zeeman effect thus became an important tool for elucidating the structure of the atom.