The Naked Scientists

The Naked Scientists Forum

Author Topic: Another question about photons hitting atoms  (Read 1741 times)

Offline Donard

  • First timers
  • *
  • Posts: 2
    • View Profile
Another question about photons hitting atoms
« on: 08/08/2011 18:43:00 »
I am trying to get head around this because I want to understand the
physics of the greenhouse effect better. I'm sure I'm missing something
very simple but what I don't understand is absorbtion spectra. If you
shine a light source on some atoms what occurs to me is that if the
atoms do not immediately emit photons following their absorption of
photons then in fact there would be no dark bands in the absorption
spectra, as the relevant electrons would be "filled up" with photons
at that frequency within a slit second and no further absorption would
take place. Am I right so far? But in fact there is an "instant"
emission (whatever instant means in this context) but it must be
either of the same frequency as the absorbed photon but in a different
direction from that of the original light (and I would have to think
in terms of a particle rather than a wave) or else a multi-stage
emission, meaning that the frequencies could be in the infra-red
band absorbed by greenhouse gases. Is what I am saying correct, or
totally confused?


 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Another question about photons hitting atoms
« Reply #1 on: 09/08/2011 00:19:55 »
You have already explained the answer to the question yourself  if an atom absorbs a photon and then emits it a fraction of a second later it emits it in a random direction from the direction that it was absorbed.  this effectively attenuates the light in the line from the source to the receiver and produces the absorption line.
 

Offline Donard

  • First timers
  • *
  • Posts: 2
    • View Profile
Another question about photons hitting atoms
« Reply #2 on: 09/08/2011 10:59:01 »
Thank you. Both the posts above and below are very helpful.
« Last Edit: 09/08/2011 16:47:58 by Donard »
 

Offline damocles

  • Hero Member
  • *****
  • Posts: 756
  • Thanked: 1 times
    • View Profile
Another question about photons hitting atoms
« Reply #3 on: 09/08/2011 13:27:59 »
All of the above is sort of right for visible light, but with infrared it is a little bit different. An infrared photon is not easy to emit, and they are on average only emitted very slowly. In fact, their rate of emission is much slower than the collision rate between the gas molecules (except at extremely low pressure). Meanwhile, it is not the motions of the fast, light, electrons that are stimulated when an infrared photon is absorbed, but vibrations of the heavy, relatively slow, nuclei.
So when an infrared photon is absorbed, it is not usually re-emitted. It makes the molecule vibrate more vigourously, and these vigourous motions lead to energy transfer in collisions. We finish up with everything moving a little faster.

The overall effect is that infrared light energy is transformed into heat energy -- kinetic energy of the gas molecules-- and infrared photons are not re-emitted in any direction.

With UV light, the effect is different again -- if UV light is absorbed, it nearly always breaks one of the bonds in a molecule, and produces a chemical reaction with high chemical energy products. It does so much quicker than any UV photon can be emitted. Light energy is transformed into chemical energy.
 

Online yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11987
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Another question about photons hitting atoms
« Reply #4 on: 10/08/2011 12:06:09 »
Very nice explanations. Heat is IR radiation specific to the molecules colliding, whether they do it in the air or are getting freed from the ground at night. It's all about 'vibrational energy' and hitting some atom with 'photons' you deliver an excess of that. Depending on the 'photons' energy, equivalent to a 'frequency' for a stream of 'photons' over time, it either will displace photons from that atom, or it will not. And that is also one of the grounds for the idea of light quanta as I understands it. But you might assume that even if there is no release of photons from that atom, as the energy, aka frequency, was wrong there still might be a kinetic energy delivered to the atom, meaning that the atom 'vibrates' a little more, even if not delivering photons in return.

And that one is tricky. I think it should transfer a momentum.
 

The Naked Scientists Forum

Another question about photons hitting atoms
« Reply #4 on: 10/08/2011 12:06:09 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums