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Does light cause electrons to constantly change state?

  1. Apr 26, 2015 #1
    Im trying to wrap my head around this. If a photon of the right energy hits an atom's electron (hydrogen e.g) it jumps to the next orbital shell, then goes back down to conserve energy releasing a photon in the process. is this how we see the atoms in everyday matter? by the switching of the electrons state from excited to ground? And if so do the electrons have to constantly change in order for the object to give of its light? this is really confusing me
     
  2. jcsd
  3. Apr 27, 2015 #2

    tech99

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    The electron does not usually need to jump to another shell. When the wave arrives, the electron just wiggles slightly in response to the electric field. In doing this, it re-radiates some of the radiation at the same wavelength. In metals, there are many conduction electrons able to move around freely. With insulators, the electrons remain "tethered", but can still move slightly.
     
  4. Apr 27, 2015 #3
    This may be confusing. And misleading.
     
  5. Apr 27, 2015 #4

    tech99

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    I was wondering if you could clarify a bit better for us what happens when the light falls on the object? Thank you.
     
  6. Apr 27, 2015 #5
    You may start by reading the FAQ on light interaction with matter.
    https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/ [Broken]
    The main idea is that the atoms in solids do not behave like individual atoms.

    But even in individual atoms I don't see what real phenomenon corresponds to the "wiggling".:)
     
    Last edited by a moderator: May 7, 2017
  7. Apr 27, 2015 #6
    I still cant understand how that determines an objects colour..
     
  8. Apr 27, 2015 #7
    Pretty sure he is referring to the influence of the EM field on the electron's motion, which in turn generates a new EM field that in combination with the original one causes refraction, slowing of the speed of light etc.
     
  9. Apr 27, 2015 #8
    It's not a single mechanism. It depends on the nature of the object and the interaction between the light and the object.
    Finding a single mode to explain color for any object is not a realistic goal in my opinion. You can try to study various cases.
    Maybe easier to understand may be color of some gems where impurity atoms embedded into the otherwise clear white crystal will behave somehow like individual atoms with discrete energy levels.

    @rumborak,
    Ah, poarization of atoms then.
    Off topic. Is your name same as that of the character from Arabela?

    @tech99
    Sorry, maybe I misunderstood you.
     
    Last edited: Apr 27, 2015
  10. Apr 27, 2015 #9

    Woooooow. In the many years I've used that handle, you are the first person to ever know where it is from :D
    I think that probably narrows down your nationality to either German or Czech.
     
  11. Apr 27, 2015 #10
    I loved that series so much....:)
    I did not ask so far because we spelled it Rumburak. I suppose yours is the original spelling in Czech?
     
  12. Apr 27, 2015 #11
    Back in the day when I chose it it was *supposed* to be the German spelling, but it was from the top of my head, so I misspelled it.
    But yes, the series were soooo good. All of them, Die Besucher etc.
     
  13. Apr 27, 2015 #12

    sophiecentaur

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    Correct. In a solid, there are not discrete energy states for an electron, due to the proximity of the other atoms. The "wiggle" is not a good description, I think, because the wiggling is not of just one electron but the whole (local) structure, for which there are a continuum of energy states. The Hydrogen Atom model is just not comprehensive enough to explain Solid State interactions.
     
  14. Apr 27, 2015 #13
    So when photons hit matter, it makes the collective vibration of the atoms vibrate more, due to the added energy and it re-emits the light and radiation back out?
     
  15. Apr 27, 2015 #14

    mfb

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    Something like that.

    Note that hydrogen gas is transparent to a wide range of wavelengths because there are so few states that could get excited. The same is true for nitrogen and oxygen, which makes our atmosphere so transparent to visible light.
     
  16. Apr 27, 2015 #15

    sophiecentaur

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    The whole notion of atoms or electrons 'vibrating' is really much too 'mechanical' to fit in with the accepted theories of what happens in the solid state. There are instances when EM waves can be thought of as causing electrons to vibrate - for instance, in the Ionosphere where radio waves propagation can be modelled pretty well in terms of fields making electrons move up and down as the EM wave progresses through the medium. The electrons are very separate from each other and can literally be treated as more or less isolated particles with mass and charge. In a piece of glass, the situation is very very different and the electrons are better treated as waves and not as little ball bearings.
    Can there be any point in ignoring this and demanding a cuddly model that fits in with a sort of intuitive (and pretty ancient) model?
     
  17. Apr 28, 2015 #16

    Simon Bridge

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    We do not see stuff by the same process that hydrogen gas can glow. You can tell this because stuff does not normally glow.

    Hydrogen is used as a starting point to teaching about atomic transitions in terms of individual electron orbitals - it's an old model and simplistic. Do not expect it to tell you everything about light. But it is still a decent place to start - so I'll start there to see if I can give you a glimpse at a more complete idea: I'll start with what you already know about hydrogen, in simple terms (@everyone else: I know - let the simple stuff sink in first OK?!!) and extend to more complicated situations - also in simple terms.

    So looking at hydrogen: the process you described only happens when the photon has the right energy (the difference between the shells) and there is usually something else to cause the de-excitation. Details depend on the exact two shells.

    An electron may be promoted to quite a high energy - though these are usually better thought of as atomic energy states rather than electron states - in which case, it may find itself headed back to the ground state in a series of transitions rather than going all in one go. It releases a photon at each transition - so a single photon in can, in principle, get a series of photons out. Some of the photons out may be in the visible spectrum - which gives the gas the color that it glows at, we can see it when it is hot or if we manipulate it via an applied electric field. When it does not glow in the visible spectrum, we don't usually think of it as having a color.

    An atom does not have to lose energy by radiating; it can also lose energy in "collisions" with other atoms, it may share energy with other atoms in the same molecule, the molecule itself may collide with other molecules stuff like that. The denser the matter, the bigger the molecule or crystal or what-have-you, the more ways the bulk object may deal with the energy on an atom-by-atom basis ... this is why the short answer to your question is "no".

    You also asked about color:

    A bulk object gets it's color primarily by absorbing a band of wavelengths in the visible spectrum. This should have been taught at secondary level but it usually needs to be repeated.

    It can do this because the structure of the object at the atomic and molecular levels allows the energy involved with those wavelengths to be favorably removed by other processes - just like the orbital structure of atoms allows it to absorb some wavelengths and not others. Usually a lot of the absorbed energy ends up as random motion of atoms - heat. It gets diffuses through the object by "collisions" - atoms pushing and pulling on their atomic bonds, that sort of thing. When the object warms up, then the energy may also get re-radiated ... at room-temperature this is usually infra-red so you don't see it so it does not contribute to the object's color. Heat it up a lot and it may radiate in the visible spectrum - hot objects do glow.

    The bottom line is: it's not that simple. The single process isolated in the hydrogen experiments you've probably seen is not the only thing that is going on. What we experience as out everyday life is a combination of many different processes - even with something as apparently straight forward as light scattering from a surface.
     
  18. Apr 29, 2015 #17

    sophiecentaur

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    I would also confirm that it cannot work as simply as one photon per electron at a time. Solids appear as solids and you get specular reflections off surfaces. If the interactions were one photon/one atom, you would just get absorption and re-radiation in all directions (as in interstellar gases) and there would be no build up of an optical 'image'. What happens is that the light behaves as a wave and interacts over a big area of the solid face to produce a coherent (specular) reflection or refraction. The notion of a single photon (little bullet) striking the surface, is not appropriate. If you insist on dealing at the photon level then you have to view each photon as having an effect over an enormous area of the solid. Sometimes waves are more fruitful to consider than photons. That implies you need to assign a photon some sort of cross sectional area?????? Not very useful as a model.
    Do not be misled into thinking that bringing photons into it is actually adding to the understanding of all situations.
     
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