Electronic transitions, forbidden radiation probability

The intensity of the band depends on the probability of the absorption, that is to say of the interaction between the incident radiation and the electrons of the system. The excitation of a molecule changes its electronic distribution and this gives rise to a dipole moment connected with the transition itself, that is proportional to its probability. A high absorption of light is possible for transitions with an high probability, that is to say with transitions that induce high variations in the dipole moment. These transitions are called allowed while low probability transitions are called forbidden and they present low absorptions values. 

---