In absorption spectrum of hydrogen atom, only one electron is present in its one atom which is in ground state, so it means that all electrons can only absorb energy of photon of wavelength which lies in UV region to get to a higher energy state (by calculation it can take max wavelength =122.55nm and minimum wavelength =91.9nm).Then why do we see dark lines in its absorption spectrum in visible region also, how can it absorb energy which is in visible region to get to a higher energy state i.e. how can it absorb wavelength like 656nm? How can the photon of this wavelength make the electron to get to a higher energy state? Please explain it intuitively (using simple mathematics)?

  • $\begingroup$ There is a misunderstanding in the question. Happy Sharma says that "we see dark lines in its absorption spectrum". This has no meaning. The absorption spectrum of H is made of thin bright lines, and has no dark lines. Dark lines may only be seen in continuous spectra like in the spectrum of the sun. $\endgroup$ – Maurice May 11 at 8:08
  • $\begingroup$ thin bright lines are seen in emission spectrum ,in absorption spectrum there are some dark lines lines which are getting absorbed $\endgroup$ – happy sharma May 11 at 8:31
  • $\begingroup$ NO ! There are no dark lines in the absorption spectrum of Hydrogen atom. $\endgroup$ – Maurice May 11 at 9:35
  • $\begingroup$ By principle, absorption spectra do have dark lines, as incoming light is absorbed, if it's energy equals the transition energy between 2 energy levels. Absorption spectrum is determined by the ratio of passing and coming light intensity ( possibly modulated ). If it had bright lines, it would mean hydrogen would absorb everything but these respective energies. $\endgroup$ – Poutnik May 11 at 17:58
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    $\begingroup$ @M. Farooq. You are right. I should have said "the emission spectra is made of thin bright lines". The absorption spectrum cannot be made of "bright" lines $\endgroup$ – Maurice May 11 at 18:52

Not a bad question but badly worded. Basically, you are wondering how can hydrogen atoms absorb 656 nm? The point is why not? Calculate the energy corresponding to 656 nm, this energy corresponds to a energy level difference in the electronic states of hydrogen atom.

Now note that you are interested in absorption spectrum. Now this is punch line. You have to do a thought experiment. Hydrogen is a diatomic gas, first you will have to provide enough energy to hydrogen that it be atomized. Now 656 nm line absorption corresponds to a transition from n=2 to n=3 as shown below.

Now you will only see this absorption when you have an electron in the hydrogen atom, which is already sitting at n=2 (this means you already have a very high temperature that that you are dealing with an excited hydrogen atom). Once this excited hydrogen atom absorbs 656 nm light, the electron will be excited to n=3.

Now no atom "absorbs" dark lines or emits dark lines. This is a poor choice of wordings. The dark lines indicate the absence or(better diminished intensity) of a certain wavelength from a continuous spectrum.

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