3p->2s: Why spectral line split into three lines in m. field

• I
HastiM
Hello,

suppose we have an excited hydrogen atom in 3p state which makes a transition to 2s state. Then the atom loses some energy by emitting a photon which can be detected (or seen). I have no problems with that situation.

If there is an external magnetic field, the energy levels of the hydrogen split due to the Zeeman effect. Moreover, people say, because of the magnetic field, the spectral lines split, too. I do not really understand why the spectral lines split? (Why do we "see" several "colors"?) In my naive understanding, if the hydrogen atom makes some transition, a single photon is emitted. That photon carries some amount of energy and thus it should correspond to a single spectral line. What is wrong with my understanding?

I would very appreciate your help.