Suppose there is a (blackbody) electromagnetic radiation source. It should emit a finite amount of photons every second with an intensity against frequency graph looking similar to a Maxwell Boltzmann distribution curve. Every photon has a specific amount of energy. Now, the source is opposite a collection of atoms of an element, for instance neon. Some of the photons have the precise amount of energy required to excite an electron and so move it up an energy level. I have been taught that the energy needed to move it up this level is exact or discrete, any more or less and the electron would not move up to the level.

Frequency - or energy - of a photon can take on any value and thus is a continuous variable. Therefore in the distribution of frequencies/energies for the photons from the source described, surely the probability that any electron has the exact amount of energy required to move an electron up an energy level falls to 0. Despite this, clearly what I have suggested is not the case because electrons clearly absorb the exact amount of energy needed to move them up energy levels all the time as evident from absorption spectra.

My question is therefore, how is it we see all this absorption if the probability that a photon has a precise energy on a continuous scale is 0? Is there some lee-way on how much energy would move an electron up an energy level?

  • $\begingroup$ Well, actual the transitions all have linewidths, so there is some leeway. $\endgroup$
    – Jon Custer
    May 15, 2019 at 16:47

1 Answer 1


I have been taught that the energy needed to move it up this level is exact or discrete, any more or less and the electron would not move up to the level.

This is an ideal statement, mostly true for an isolated atom. But even when true, it doesn't mean that other interactions are absent (such as scattering or ionization).

First of all, the atom "sees" the frequency of the incoming radiation differently depending on its speed. Interactions with other atoms can affect the process as well. So at high temperatures and high pressures, the range of frequencies that can be absorbed by a single atom increases.

More important for everyday experiences is that molecular electron configurations (especially as the molecular size increases) are significantly more complex than isolated atoms. The interactions of the electron shells mean that that the discrete, easily-detectable energy levels disappear with wide ranges of absorption possible.

The nitrogen, oxygen, argon in our atmosphere is a good example of simple molecules that have trouble absorbing a wide range of light. But once you create bulk matter, the possible absorption goes up and it becomes harder to find materials that will pass large frequency ranges.

  • $\begingroup$ $+1$, and I would add that even a isolated single-electron hydrogen atom has nonzero emission/absorption linewidths. It doesn't when you compute its energy levels using a model that neglects the quantum EM field, but when the quantum EM field is included (which is a prerequisite for emitting radiation anyway), the linewidths are nonzero. This is called the "natural width" on page 73 in Cohen-Tannoudji et al (1992), Atom-Photon Interactions $\endgroup$ May 16, 2019 at 1:57

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