I am reading about this apparatus at Wikipedia: https://en.wikipedia.org/wiki/Magneto-optical_trap And there is the following sentence which I don't understand: "As atoms travel away from the field zero at the center of the trap (halfway between the coils), the spatially-varying Zeeman shift brings an atomic transition into resonance which gives rise to a scattering force that pushes the atoms back towards the center of the trap." I am familiar as to how the Zeeman effect affects the energy levels in a particle with and without spin. For a particle without spin we have 2l+1 sub levels for a given l value. When spin is taken into consideration (I don't know whether is the spin of nucleus or of the electrons) we have 2J+1 sub levels for a given J value. But I don't understand the part in bold. What does it mean to bring an atomic transition into resonance? What is the atomic transition? The superposition of the two states where one is the initial state and the other the final? Can the Zeeman effect split superposed states? And if yes, how many splitting (I am sorry for the vocabulary here) occur? I am confused with the above statement.