# Why 3 lines in Zeeman effect?

Why is there a central line when a magnet is used to split the electrons? I would have thought that since electrons have two characteristic states that one type would go one way and the other type the other way so the middle should be blank.

Khashishi
In the case of the normal Zeeman effect, we don't need to consider electron spin. Then you only need to care about the z component of angular momentum quantum number m. The allowable transitions in LS coupling have delta m = {-1, 0, or 1}. This gives you three lines.The presence of the magnetic field causes the energy of an "m" state to be shifted by $\mu_B B m$. This energy shift happens to both the upper levels and lower levels of the transition, so only the change in m matters.

Thanks for answering but I don't understand why the middle line is present. I have read about the Zeeman effect here
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/zeeman.html
but none of this explains what the middle line represents so can someone explain it in simple English for me.
Does the middle line say that some of the electrons are not affected by the magnetic field and so just go straight? If so why aren't they affected?

It depends on the model you are using. There will be an odd number of lines if you do not take account for the spin of the electron, because the different angular momentum states of the electron will be odd (2L+1, where L is integer). In this case, the middle line is for m=0, i.e. for the state whose energy is not affected by the magnetic field. If you take account for the spin, then there will be an even number of lines because there is an even number of angular moment states (2J+1 , where J is half-integer) . Here, there is not a central line because there is not a state with Jm=0.

The electron has a magnetic moment therefore has to be affected by the applied magnetic field so how can there be a middle line?

Have you performed an experiment and you found three lines?
Or it was somewhere else that you saw these lines? If yes, these was experimental or theoretical data? If theoretical, then did you check what model the author used? Did he/she take account for the spin of electron?

Khashishi