The Zeeman Effect Apparatus examines the effect of a strong magnetic field on the green emission line of mercury at 545.1nm wavelength.
A magnetic field splits the degenerate 7s and 6p levels into three and five levels respectively, giving nine allowed transitions.
The energy shifts of the splitting are tiny compared to the transition energy, so a high resolution spectrometer is necessary to observe them at the magnetic field strengths achievable in the laboratory.
This resolution is usually achieved by using a high quality Fabry-Pérot étalon.
The combination of these requirements has often discouraged the provision of Zeeman effect apparatus in the student laboratory.