Let's say we have a transition from state 2p to 1s of an hydrogen atom's electron. The intensity radiated by the electron is given by I = 4/3 ωfi4 e2/c3 |< f | r | i >|2.
If we take the the | 2 1 1 > → | 1 0 0 > transition for example, we must compute the following integral :
< f | r | i > =...