Hi, I am trying to understand how one determines the natural linewidth. On my assignment, I am only given an energy (589.1 nm transition in sodium). I have two sources that I have found that seem to contradict each other: Source 1: Slides 5 and 6 Source 2: Hyperphysics If I plug in either the lifetime or the energy value provided in the example from the powerpoint (slide 6) into the Hyperphysics calculator, the corresponding value isn't consistent with slide 6. I am confused on which is the correct formula, as well as how one determines the natural linewidth without knowing the lifetime of a transition. Is the energy of the transition actually relevant? Can anyone please advise? Thank you for your time and help.