Absorbtion lines in spectrum question

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Discussion Overview

The discussion centers on the phenomenon of absorption lines in the spectrum of light passing through a gas of vaporized elements. Participants explore the mechanisms behind the creation of these dark lines, the behavior of photons during absorption and re-emission, and the differences between various scattering processes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that shining white light through a gas creates absorption lines, which are unique to each element, but questions why these lines appear given that electrons quickly re-emit absorbed photons.
  • Another participant suggests that the re-emitted light is dispersed in all directions, implying that only a fraction of it reaches the observer's eyes.
  • A participant raises a question about the differences between absorption and scattering processes, specifically mentioning Compton scattering and its relation to ionization.
  • One participant proposes that Brewster's angle may determine the direction of re-emitted light.
  • Another participant clarifies that absorption and re-emission (fluorescence) occur on different timescales compared to scattering, noting that the atom can be measured in an excited state during fluorescence.
  • A later reply discusses the spherical symmetry of photon emission from an individual atom, stating that photons are equally likely to be emitted in all directions unless external factors break this symmetry.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms of light emission and the distinctions between absorption and scattering processes. The discussion remains unresolved regarding the specific factors that influence the direction of re-emitted light.

Contextual Notes

Participants mention various scattering processes and their differences, but the discussion does not resolve the complexities of these interactions or the assumptions underlying the explanations provided.

AStaunton
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shining white light through a gas of a vapourised element causes absorption lines to be created in the white light spectrum ie. there are dark lines running through the otherwise continuous spectrum and the position of these lines are unique to particular element.


Why is it though that we see these lines at all considering that when an electron absorbs a photon it and goes into an excited state it is only there for a very short period of time before it re-emits a photon of that wavelength again, ie an atom absorbs the photon but then re-emits it very quickly after. I can't see then, why the effect is that we see dark lines.
 
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The re-emitted light is in all directions, so only a small part of it actually ends up entering your eyes.
 
jtbell said:
The re-emitted light is in all directions, so only a small part of it actually ends up entering your eyes.

While I know (I think) that Compton scattering and this are different, how is it that they are different?
 
what determines the direction of the reemitted light?
 
The Brewster's angle i think!
 
khemist said:
While I know (I think) that Compton scattering and this are different, how is it that they are different?

Compton scattering involves ionization. Scattering of light without ionization is Raman or Rayleigh scattering.
Absorption and re-emission (fluorescence) is different from scattering because it occurs at an altogether different timescale. In short, you can measure the atom to be in the excited state during fluorescence, but not during scattering.
 
AStaunton said:
what determines the direction of the reemitted light?

For an individual atom, which we're talking about here, you have spherical symmetry. The photon is equally likely to be emitted in all directions.
If there's something around there to break that symmetry, then the situation becomes different, and it'd depend on which states the transition was between.
 

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