Question

In terms of atomic physics. What does the term 'selection rule' mean, and why do some weak spectral lines arise from transitions that violate selection rules?

Answer 1

Selection rule in atomic physics refers to the transition rule that compels the possible transitions from one quantum state to another for a system. These are the set of restrictions that govern the probability of a physical system getting changed from one state to another or else no transition will take place.

Selection rule describe allowed transitions that have a larger possibility of taking place or forbidden transitions that have less or no possibility of taking place.

Some weak spectral lines arise from transitions that violate selection rules because, at higher temperatures, electrons are found throughout the entire energy level structure and scarcely visit second energy levels, resulting in weak lines.