# Can two spectral lines from different elements be equal?

I'm not asking about how emission and absorption lines are created, nor am I asking why different elements have unique spectral "fingerprints". Those two questions have already been answered multiple times.

My question is simply if there is any fundamental reason why we couldn't find any pair of elements that have a single spectral line in common?

As a second question; if there is no reason why this couldn't be the case, have we actually found a example of this in nature, or is it just theoretically allowed?

• -1. No research effort. Google "spectral lines of elements" and you will find astro.u-strasbg.fr/~koppen/discharge at the #1 hit. Commented Sep 5, 2017 at 10:40
• @sammygerbil I don't want to be rude but I fail to se how that page answers my question? Commented Sep 5, 2017 at 10:57
• It shows the spectral lines of dozens of elements. You can see at a glance which spectral lines coincide. Isn't that what you are asking? Commented Sep 5, 2017 at 10:59
• If you reject two things being equal when they are arbitrarily close, then you are rejecting measurement as a method of deciding if two things are equal. This would make your 2nd question unanswerable, because we cannot measure anything with infinite precision. We cannot tell the difference between two lines which are theoretically identical and two which are only too close for us to distinguish the difference. Commented Sep 5, 2017 at 11:20
• Granted my second question might be "practically" unanswerable if I require infinite precision but it could still be "theoretically" answerable. I mean if you read @emilio-pisantys answer below it basically answers all my questions perfectly. I'm (obviously?) just a random person on the internet with an interest in physics and not a fancy double physics master like yourself so I'll apologise for my less then satisfactory use of proper terminology :P I hope we can agree to disagree but from "down here" my question and the given answer are still interesting :) Commented Sep 5, 2017 at 11:33