Hi I'm looking at the structure of the Zeeman effect on the two yellow lines in the sodium spectrum. I'm using a Fabry-Perot etalong and a sodium lamp. The thing is, when I have just started the lamp, and don't have a magnetic field applied, i only see the two rings expected from the fine structure. After a while, one or both of the rings split up. I suspected that heat had something to do with this, and turned up the voltage of the lamp. After a minute or two the rings had totally smudged out almost to a continuum, but with a dark spot in the middle. My explenation to this is that doppler broadening is causing the smudging, and that the dark spot now is caused by absorption of the same sodium gas. I overlapped the images from a cool lamp and a hot one, and it looks like the dark spots of the hot spectrum is at the same place as the bright spot of the cool spectrum. Is this explanation correct?