Absorbtion lines in spectrum question

[AStaunton]

shining white light through a gas of a vapourised element causes absorbtion 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.

 

[jtbell]

The re-emitted light is in all directions, so only a small part of it actually ends up entering your eyes.

 

[khemist]

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?

 

[AStaunton]

what determines the direction of the reemitted light?

 

[oraclelive]

The Brewster's angle i think!

 

[alxm]

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.

 

[alxm]

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.