Plasma Color: What Makes It Blue & What Colors It Can Be?

[fawk3s]

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

 

[JaredJames]

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

CF4: blue
SF6: white blue
SiF4: light blue
SiCl4: light blue
Cl2: whitish green
CCl4: whitish green
H2: pink
O2: pale yellow
N2: red to yellow
Br2: reddish
He: red to violet
Ne: brick red
Ar: dark red

 

[fawk3s]

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

 

[Drakkith]

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.

 

[Kracatoan]

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.

 

[JaredJames]

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.

 

[fawk3s]

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.

Thats interesting. Hadnt heard of that at all.

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.

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.

Personally, I am 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 wouldn't 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?

 

[JaredJames]

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.

 

[Borek]

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.

 

[JaredJames]

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).

 

[Borek]

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.

 

[JaredJames]

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.

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.

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.

Quite possibly. I don't suppose we have examples of 'lightning' in other gaseous compositions?

 

[Borek]

Quite possibly. I don't suppose we have examples of 'lightning' in other gaseous compositions?

Other than Earth atmosphere? I think we do.

http://www.nasa.gov/centers/goddard/multimedia/largest/lightning.html

 

[Nakisima]

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

 

[Drakkith]

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

 

[Borek]

Could the heat of lightning produce something around it to refract light?

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

 

[Borek]

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

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.

 

[fawk3s]

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 don't even know how much, if any blue wavelength would make up that particular light of the plasma.

 

[Drakkith]

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.

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?

 

[fawk3s]

I believe most of it is plasma, as otherwise there wouldn't 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.

 

[Drakkith]

I believe most of it is plasma, as otherwise there wouldn't 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.

When the gas is ionized and loses some number of electrons, do the electrons release visible light when they re-bind to the atom and lose energy?

 

[Nakisima]

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

Not to change colour, for pure light to be split into it's spectral colours. Like water refracts light to make it look blue-green.

 

[Drakkith]

Not to change colour, for pure light to be split into it's spectral colours. Like water refracts light to make it look blue-green.

Sure, but that doesn't happen in a plasma. The emitted light is actually only certain frequencies. If it was merely refraction then moving to different spots would give you different colors.

 

[Borek]

Sure, but that doesn't happen in a plasma. The emitted light is actually only certain frequencies.

As explained many times over - depends on the mechanism of light emission.

If it was merely refraction then moving to different spots would give you different colors.

Not necessarily. A lot depends on the geometry of the source and the refracting media. Prism is designed to give a rainbow.

 

[fawk3s]

When the gas is ionized and loses some number of electrons, do the electrons release visible light when they re-bind to the atom and lose energy?

I believe that's what makes plasma emit light. The positive ions, depending on the gas, are not always "electron free" either. As free electrons bump into them, they give it enough energy to boost an electron to a higher orbital and when it drops back to the lower orbital, it releases energy in the from of a photon. I guess its the case in some gas discharge lamps, but the regular ones, like fluorescent lamps, use some extra materials in the tube like mercury. The electrical current changes mercury into vapor and upon collisions with the ions mercury releases UV light. The UV reacts with the phosphor coating of the tube which releases visible spectrum photons.

 

[Antiphon]

Gas pressure has a lot to do with it too. Low pressure discharges tend to emit transitional lines whether electronic or molecular. Increasing the gas pressure causes the molecular lines to broaden.

Temperatures are indeed high for a high power arc like lightning which is why it looks blue-white. It's mostly black-body emission at a very high Kelvin temperature. And that's why the sprites and auroras are colored. Low pressure, low current density discharges. No longer thermal but electronic and molecular.