system
Given the following post, respond as if you're commenting on Reddit:
human
Photons of a specific frequency are absorbed by atoms. How precise must that frequency be?

I was taught that certain elements' electrons absorb and emit photons of specific frequencies.  Well, how specific must the frequency be?

Suppose an element X absorbs photons of frequency F.  Would the atom absorb photons of frequency F+1?

Given that atoms are moving / rotating / not actually in one place at one time, how does this absorption even occur?  Wouldn't doppler shift cause the frequency to not ever be _exactly_ F?  Does the photon have to hit directly?
gpt
Good question. There will always be a finite width to the absorption lines. This can be caused by the motion of the atom, which will doppler shift the photons a bit. That is generally the largest effect for atomic spectra, but if you could reduce the doppler broadening there would still be a finite width caused by the uncertainty principle, which you can roughly estimate as the Planck constant divided by the lifetime of the excited state.