I was reading a book (Sears Zemansky) about this subject but I didn't understand something of an example, and this is that according to me there should be a process of emission for each of absorption (and vice versa), or am I wrong?.
In the example the statement says that a hypothetical atom has 3 possible levels of energy, the fundamental, 1eV and 3eV, with the following possible transitions.
But why there is not the transition from 1eV to 3eV?.