why rotational in vibrational?? Hello, I recently had an exam in Molecular physics, in which one of the unanswered questions asked by my proff was "How come there are rotational transition lines in a vibrational spectrum? because we observe vibrational spectrum in infrared level radiation and rotational in micro wave level ,,then how rotational lines are incorporated in vibrational spectrum"?? please let me know because I have to give this exam again!!!