I'm quite confuse with some concepts here. The selection rules are derived from the requirements that quantum numbers must be conserved. It's OK. Then I see they give rules for so-called electric dipole transitions. I just wonder why dipole comes in here. How do you classify these kinds of transitions: 1. An electron absorbs a photon and then jumps to the higher level. 2. In magnetic resonance, if we apply an rf with energy match to the separation between two energy levels, the electron spin can flip. The difference to case 1 is we have magnetic field involved. Can you give me the examples of dipole-dipole, magnetic-dipole, electric quadrupole, magnetic quadrupole, quadrupole transitions? Is there any way to understand the selection rules rather than remember the table of selection rules?