We know absorption spectrum of sodium is near 595 nm wavelength which is in the region of yellow colored light. Why doesn't sodium (valence electrons) absorb EM wave of wavelength smaller than the wavelength 595 nm? I know that it cannot absorb wave of energy less than the two energy levels, but it can absorb more than that. By the way is it because the other wavelengths don't comply with energy required to move to any other energy levels?

I am looking for another clarification. That is if sodium absorbs yellow light (suppose) and releases it within a very short period of time then how do we observe the gap in the spectrum? Or is it the first set of incident light on photodetector where we observe that gap only?

Edit: I forgot that waves of lower wavelength possess higher energy. So previously I mentioned "larger" instead of smaller


1 Answer 1


In fact, it does. The famous yellow double line of sodium is the brightest, but there are a lot more of them.

enter image description here

Source: wiki article for Na

Sodium has 11 electrons and few readily available ionization states that offer quite a diversity of transitions.

p.s. if you ask about why sodium atom cannot absorb a part of the energy of the photon, corresponding to its own transition and leave the rest as another, lower-energy photon - it does that, too, sometimes.

It just likes its own spectral lines much better. That's what the resonance is all about.

  • $\begingroup$ Why is that they are absorbing rays of larger wavelength than 595nm according to the image? I mean in the red region. $\endgroup$
    – MSKB
    Commented May 10, 2021 at 15:02
  • $\begingroup$ See, e.g. here: hyperphysics.phy-astr.gsu.edu/hbase/quantum/sodium.html - and the page discusses only the valence electron of a neutral sodium. It has quite a few levels to jump between. Well, you may need to kick it into something else than its ground level first. $\endgroup$
    – fraxinus
    Commented May 10, 2021 at 15:10
  • $\begingroup$ @Scintron1497 The spectrum shown is an emission spectrum not an absorption spectrum. Just saying. The absorption spectrum would usually appear to have fewer lines as it would mostly consists of principal lines, that is those originating from the ground state (only the ground state would have enough electrons in it to cause noticeable absorption). Also, this is a theoretically computed spectrum not an observed one. $\endgroup$
    – Thomas
    Commented May 10, 2021 at 21:21
  • $\begingroup$ Thanks a lot @Thomas and@fraxinus $\endgroup$
    – MSKB
    Commented May 11, 2021 at 9:44

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