I am trying to understand selection rules in atomic transitions. So, one of the selection rules says that only transitions where orbital quantum number is changed by 1 are possible. If that is the case how can hydrogen in ground state get to 2s state? Can we detect spectroscopic line that corresponds to 2s->1s transitions in H atom? PS. I used "possible/impossible" where instead I should have used "probable/improbable" as selection rules don't forbid states just make them improbable (though in my book this is not stated, I found this online). So in that case, answer to my question is "yes but it's improbable". But in my course we exercise problems and assume that selection rules really do forbid certain transitions. Anyway, that is not main point of my questions but it would be nice to know in what case selection rules are "broken" and improbable transition happens?