I learned that a hydrogen lamp emits photons at specific spectral lines because of electrons falling from a higher energy level to a lower energy level.
I think I understand this.

However, nowhere can I find an explanation of the following, deeper, questions:

  • Isn't the hydrogen electron sitting at the lowest energy level and therefore it first needs to be brought up to a higher energy level? Is this achieved with the higher voltage? If so, why is the voltage resulting in this? Is there an electron flow from plus to minus when the voltage is applied?

  • I think that if the voltage brings the electrons to a higher level, then the voltage needs to disappear so that they can fall back and photons are being transmitted. This leads me to believe that we need AC current for this, which results in a periodic change of the energy levels of the electrons, and thus a periodic emission of photons - is that correct? So this would not work with DC?


  • $\begingroup$ You may want to read about He-Ne laser, which works in a rather similar way. Although it is a bit more complex than a hydrogen lamp, the explanations in the Wiki article are clrearer: en.wikipedia.org/wiki/Helium%E2%80%93neon_laser $\endgroup$
    – Roger V.
    Commented Jun 8, 2021 at 8:31

1 Answer 1


(a) In a hydrogen discharge tube a very small fraction of the hydrogen atoms will be ionised (e.g. because of 'background' ionising radiation). The high voltage accelerates the ions, and if the voltage is high enough, or the gas density low enough, they will acquire enough kinetic energy before making collisions with atoms to excite those atoms, in other words to raise them into higher energy states.

(b) An excited atom 'relaxes' spontaneously into a lower energy state, emitting a photon; there is no need for the voltage to be switched off. The voltage doesn't excite atoms directly, but only by the collision mechanism described in (a). The chances of an excited atom being hit again, before it has relaxed, by an accelerated ion, are negligible.


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