The color of plasma

  1. Most of the plasma we see in everyday life is more or less blue (lightning, plasma lamps, static discharges through air etc etc). But if I understand correctly, plasma can be in many colors. Like the plasma atmosphere of the sun, which as far as I know, is not blue.
    What makes the usual plasma glow blue? What does the color of certain plasma depend on? Also, in which colors (overall, in vague) can plasma appear? And what color is the suns atmosphere, or the several stages of it?

    Thanks in advance,
    fawk3s
     
    Last edited: Jan 6, 2011
  2. jcsd
  3. Plasma colour is determined by the type of gas used to produce it.

    Here is a list of colours produced by various gases:

    http://www.plasma.de/en/glossary/glossary-entry-486.html
     
  4. If regular air is mostly made up of N2, O2, H2O and smaller amounts of other gases, then how come the plasma glow we see during a static discharge or lightning is blue? Because that list seems to state that a mix of O2 and N2 would give you a orange-ish color. Or am I wrong about the consistency?

    Thanks in advance,
    fawk3s
     
  5. Drakkith

    Staff: Mentor

    Does it have something to do with the temperature of the air? I've read that the temperature of lightning is somewhere between 18,000 and 54,000 degrees F, which would put it at least white or blue hot? Just throwing out a guess here.
     
  6. Also, I think with really high temperature plasmas, like in stars, colour depends on temperature, allowing us to estimate a star's temperature from its colour.
     
  7. Lightning varies in colour.

    In the upper atmosphere where you get jets and sprites the colour can be anything from red-orange to green to blue.
     
  8. Thats interesting. Hadnt heard of that at all.

    Personally, Im leaning toward the temperature solution aswell. Lightning for one has a pretty high temperature, and so does the electrical arc of static discharge. Not sure though.
    I wouldnt know much about star temperatures, but as sun is said to appear clear white in space, then does that mean the atmosphere/corona of it is white aswell?
     
  9. View the sun through an camera that sees a different spectrum to us (x-ray, thermal, ultra-violet) and it will appear a different colour each time.
     
  10. Borek

    Staff: Mentor

    I guess color is a function of temperature and changes in the mechanisms responsible for emissions. When plasma is hot enough, it will behave more or less like a black body. When it is "cold", most light is emitted by excited atoms/molecules, so the color is more dependent on the composition.
     
  11. Well, I've done some reading and most seem happy it's the heat. But not everyone agrees with it being the composition of the gases (assuming that's what you meant by the composition Borek).
     
  12. Borek

    Staff: Mentor

    In gas-discharge lamps gas that emits the light is ionized, at least partially, so it qualifies as a plasma. But the lamp color depends on the gas used (that's what I was aiming at writing about the composition). On the other hand, when gas gets really hot (like in stars), what we observe is more or less black body radiation. So we will get blue light from mercury at few hundreds K, or any gas at 20 thousands K.

    Could be the question is a little bit ambiguous - it doesn't state why the plasma emits light. Hot plasma emits light because its hot, mercury vapor emits light because it is artificially excited.
     
  13. That seems the general point being made. The light is due to heat as opposed to the gas composition that is ionised as the colour doesn't necessarily follow the composition.
    Quite possibly. I don't suppose we have examples of 'lightning' in other gaseous compositions?
     
  14. Borek

    Staff: Mentor

  15. What about light refraction to determine colour? Could the heat of lightning produce something around it to refract light?
     
  16. Drakkith

    Staff: Mentor

    How is light emitted from a lamp compared to plasma in say the sun? Are both simply thermally excited, or is a lamp different?
     
  17. Borek

    Staff: Mentor

    For sure it does, there must be a density gradient between hot gas and cold gas around. But refraction doesn't change color.
     
  18. Borek

    Staff: Mentor

    Depends on the lamp. Incandescents - while don't contain plasma - emit the light just because wire is hot. In gas discharge lamps atoms (or molecules) get excited by collisions with electrons or other ions/atoms that were accelerated by the electric field between electrodes, then they emit photons when falling back to lower energy states. The difference can be easily seen in the spectra - one is continuous, other is discrete.
     
  19. I think refraction would be highly unlikely, because only the blue part of the light would be reaching all of the observers eyes. In this case, we also dont even know how much, if any blue wavelength would make up that particular light of the plasma.
     
  20. Drakkith

    Staff: Mentor

    So in a gas discharge lamp, only a small portion of the gas (if any) would be a plasma while most of it is still in atomic/molecular form?
     
  21. I believe most of it is plasma, as otherwise there wouldnt be much point to the gas, as it would add resistance in the tube. As most of it is ionized, the more frequently the mercury gets excited and emits light.
     
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