Why are sparks blue?

I am not sure if First Excited State posted this as a blog entry, but it was mentioned on twitter. Question: why are sparks blue? My first gut response was that this is the blackbody color. Wrong for several reasons. The short answer is that sparks are blue because of the colors given off from nitrogen and oxygen when they are excited.

In order to make this post longer than necessary, let me say something about blackbodies. A blackbody is an object that emits radiation due only to it's temperature. Since it does not reflect anything, it looks black at room temperatures. You can make a black body, it's not hard. Simply take a closed box with a small hole in it. Look at the hole, it will appear black regardless of the actual color inside. Here is an example of one I made. Ok, I can't find a picture of that box. I will post it later because it is pretty cool. Instead, here is a diagram:

Essentially, light goes in, but doesn't come out (like thunder dome). When light goes in, it reflects off the surface but some of it gets absorbed. Each time it reflects, some gets absorbed. By the time it finally makes it out of that tiny hole, there is essentially nothing left. What does come out of the hole is light that is produced by the thermal activity of the material (and not by reflected light). It looks black to you because all of this blackbody radiation for this temperature is in the infra red spectrum.

Some other examples of blackbodies that you are probably familiar with:

• Incandescent light bulb filament while on.
• The sun (while on).
• A hot stove element.

All of these objects give off radiation that is related to the temperature of the object. The higher the temperature, the more light given off at shorter wavelengths. These objects actually give off radiation (note that I am using light and radiation interchangeably) at essentially every wavelength. This is usually called a continuous spectrum. If you looked at it through a spectral slide or a prism, you would see all the colors of the rainbow. The best way to see this is with this awesome applet from PhET.

Blackbodies and other types of radiation are very complicated (quantum mechanically speaking). What is the difference between blackbody radiation and other stuff that gives off light? If you looked at a fluorescent light through a spectral slide, you would not see the rainbow. Instead you would just see some colors. If you have not done this before, you should get one of these spectral slides or glasses. They really are cheap. Just don't use it to look directly at the Sun (regardless of what Phil Plait says because it would suck if he was wrong). This is usually called an emission line spectra (as opposed to continuous)

What is the difference here? An emission line spectra is created when there is an excited gas. By excited, I mean that the electrons in the gas jump up to higher energy levels, and then fall back down. When they drop down they give off light. The frequency of the light produced is related to the change in energy levels. That is as much detail as I want to go into here, but if you are interested, see this post. So, different gases have different energy levels and thus produce light of different frequency.

Why don't blackbodies do the same thing? How come the light only depends on temperature and not the material that it is made of? (for example a gas of excited iron vs. a block of iron) The reason is that the energy levels in a block or iron are completely different than the energy levels in atomic gas of iron.

Ok. Back to sparks. The light could not be blackbody radiation because it's a gas. The light is actually given off when free electrons recombine with air ions (air ions means oxygen or nitrogen molecules missing an electron). To examine the spectra from a spark, I am going to put one of these spectral slides from Educational Innovations and put it in front of my video camera. Then I can use Tracker Video to analyze the spectrum. Here is a picture of the same thing with Hydrogen gas.

And using tracker, I can get the intensity of the light along that purple line I drew in there.

Now for comparison, here is the same thing done with a spark.

And here is a graph of the intensity.

No analysis, but that does not look like a continuous spectrum.

Finally, some other interesting things about sparks (for more details on this, see the excellent analysis of sparks in Matter and Interactions Vol II by Chabay and Sherwood).

• A spark occurs in air in the electric field exceeds 3x10⁶ Newtons/Coulomb.
• It is NOT because charge is jumping from one object to the other.
• Free electrons in the air are accelerated in the opposite direction to the electric field. These electrons collide with molecules and free other electrons creating an electron avalanche.
• The light comes from electrons recombining with air ions (as stated above).
• The electric field is not strong enough to pull electrons from the air molecules. These electrons had to already be there. (and they are from radioactive sources and cosmic rays).
• In a vacuum, you wouldn't see a spark (no air). Also, no one can hear you scream. (I know I keep using that joke, I am sorry).

As a final plug for Matter and Interactions they have an order of estimate calculation for how large an electric field would have to be to accelerate electrons to the speed that they knock out other electrons. They compare this to the experimental value of 3x10⁶N/C. Cool.