So I am doing a practical on the Zeeman effect, and have not been able to find an answer to the following: The experiment used a cadmium lamp, with it's light going though a fabry-perot interferometer. When the magnetic field was applied perpendicular to the direction of the interferometer, this was observed: The peak has split into three, because of the splitting of the energy levels, and so photons are produced of three different energies. When the magnetic field is applied paralell to the interferometer, (there are holes in the magnets for light to shine through), the peak splits up like this: where the central peak disappears, and only the two side peaks are present. Why does this happen? I think it's something to do with how the light is polarised, but I don't understand how the different direction in magnetic field can do this? EDIT: if it's not obvious, each image shows an increase in magnetic field, from 0-~0.6T.