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What’s a fire, and why does it burn? (qchu.wordpress.com)
276 points by pizza on Jan 10, 2017 | hide | past | web | favorite | 92 comments



When I was a volunteer firefighter circa 1999 we were forced to learn the definition of fire that would appease a lawyer / court in case we got subpoenaed.

Apparently somewhere else there had been an arson fire and the fire chief was on the stand and the defense asked him what fire was and all he could come up with was "it's hot" or something similar. I don't know if it caused the case to be won or lost but our Chiefs decided we wouldn't chance the embarrassment.

I'll now forever remember that "fire is the rapid oxidation of a combustible material resulting in the release of energy in the form of heat and light".

Still waiting on that subpoena and my chance to put this to good use. Or at least trivia night or something...


I was also a volunteer firefighter around those years as well but we had to learn the definition of fire using the fire triangle [0]. It is a simple model to reason about fire and how to extinguish it: just remove any of the three components (oxygen, heat or fuel).

[0] https://en.wikipedia.org/wiki/Fire_triangle


Halon fire extinguishers are interesting in that they disrupt the chemical reaction of the fire directly. There's still fuel, heat and oxygen, but they just don't react.


When I was an intern, I worked in IT for a Fortune 500 company. They'd recently decommissioned their old tape loaders for the mainframe and had moved to an automated tape silo, so no longer had need for the tape room. They converted the tape room that was connected into a conference room, but they didn't disconnect the HALON. Although, smoking wasn't allowed in the facility anyway (this was early 2000s), they always made a point to remind us visitors (the data center was a several hour drive from corporate) to not smoke, and if the alarm went off, you had about 10 seconds to exit the room before the door automatically shut, with no way to exit from the inside and all oxygen would be evacuated from the room, and you would die a horrible, horrible death.

I always tried to get a seat as close to the door as possible.


Locking doors and oxygen removal would have been just a fun story. Halogen gas, at a concentration needed to disrupt a fire, is only mildly dangerous and you don't need to remove the air when you are using halogen. The halogen does not work by displacement but actually disrupts the combustion process. No need to lock the doors.


And that is a great example of Spolsky's Law of Leaky Abstractions.


Although to be fair the link that was provided also adds the "chain reaction" in a fire tetrahedron.


And fun/scary fact- they DON'T work on lithium ion batteries... I think it might slow it down but it doesn't stop them.

https://batterybro.com/blogs/18650-wholesale-battery-reviews...


That's what I was taught in the Navy for my various firefighting courses - along with types of fires, how to handle the various types, and so on. Very useful stuff.


That reminds me of how one of my Physics instructors made us memorize Archimedes' principle:

The buoyant force on an object is equal to the weight of the displaced fluid.


to be honest I don't think that this article answers the question 'what is fire' but more 'what exactly is the flame of a fire'


There is mention early of fire in the technical sense, before moving on to considering flame in greater detail. For my part, on reading the headline I expected and hoped that the discussion was about flame. Naming things is hard.


Isn't the flames the fire? If you only have embers, you wouldn't call it a fire.


Can you have a flame without the fire? You certainly can have something very similar. Personally, I think those names are all confusing and ill-defined.

Does "fire" imply in sustainability? Does it need to be combustion? Does it even have to be chemical?


> If you only have embers, you wouldn't call it a fire.

Well, you might

https://en.wikipedia.org/wiki/Coal_seam_fire


You can certainly have fires without flames. Particularly notorious is hydrogen, which burns cleanly with no flame (just heat ripples). This is one of the major challenges for hydrogen fueled vehicles:. First responders need to be aware that the combustion properties for a hydrogen vehicle are different and could be deceptively more dangerous than their trained instinct.


Heat and radiation, some of which is called light. Infrared energy isn't really heat, nor is it generally called light as it isnt visible.

Similarly, many chemical fires release substantial amounts of UV, but i doubt anyone would see much of it in an emergency through all the smoke and ir. Think rocket fuels.


I've frequently seen "light" used to refer to the entire electromagnetic spectrum in physics discussions.


Yeah, I typically hear visible light referred to as, well, "visible light". It may depend on the whims of the particular textbooks and professors you learned from, but in my physics classes we always used "light" to refer to the entire electromagnetic spectrum, and I only realized relatively late in life that some people don't use the terminology that way.


Plus I like to think of all light as being "visible" to some material or apparatus. Our eyes just happen to be tuned to a particular part of the spectrum. You don't even have to look very far to find other animals that can see wavelengths we can't, like ultraviolet. Radio antennas "see" radio waves, film emulsions and CCDs "see" X-rays, etc.

Also with radio we talk about different materials being "transparent" to one wavelength or another.


If you ever want to have some fun flying, carry some lead crystal in your carry on luggage. A number of years back, I was visiting my parents for Christmas, and they gave me a set of lead crystal tumblers. Apparently due mostly to the lead and partly to the cut of the tumblers, they appeared as hand grenades on the luggage scanner. The look of absolute puzzlement on the TSA agents' faces were priceless. They could not reconcile what they saw on their scanners with what they held in their hand, even when I explained what it was. I've had similar experiences flying with lead crystal ornaments. Which, while always a gift, I never have wrapped while I'm flying, because I know, inevitably I'd need to open it, anyway.


That's great! I wouldn't have helped them! I would have just said "Those are drinking glasses."


It's amazing to think that, for the wifi module, the rest of the laptop is more or less transparent, as in a crystal case.


And ive seen oxygen and argon called "metals" (in stars). Each field has thier own terms.


Well the metallic state has a pretty precise definition, presumably the oxygen in stars exists in that state. (Whereas oxygen in air does not)


Nope, not at all. In astrophysics, "metal" refers to any element heavier than helium, regardless of oxidation state. It is a useful definition, because only hydrogen and helium were present from the big bang. The metallicity of a star then determines the age of the nebula that formed it. More metals (in the astrophysical sense) means that other stars had to exist, make heavier elements, and explode prior to that stars birth.

It's a different definition, but it is quite useful in the field.


Again, "the metalic state" is for a different field. Same root words, different definitions and uses depending on context.


Physicists (even those who work in optics) routinely use light to mean anything on the EM spectrum, so I'm not sure where you're getting the "right" technical definition.


Because this is a firefighter's definition for use in a courtroom. It isnt about physics but the realities of flame. In the real world there are things like "invisible fire", common at racetracks during the day, that need to be accounted for. Tge physics of stellar astronomy do not.


"nor is it generally called light" sure doesn't sound like it's talking about the specific, legally relevant definition used by firefighters.

Also most physicists whose work is centered around EM radiation aren't astrophysicists and plenty of them work specifically with visible light, so your patronization is off the mark.


Most physicists also do not have much experiance with uncontrolled fire. To say that all fires produce light will be confusing if trying to explain non-visible flames to a lay jury.


If you want to be pedant, the energy from the combustion is released in the form of heat (some of which is called radiation (some of which is called light)).


Eh. Heat is energy with certain entropic characteristics. Electromagnetic radiation carries that energy, but is not itself the energy.


I was paraphrasing a previous comment. Heat is the energy transferred between systems and that's how I interpreted the "energy is released in the form of" in the original comment.

I'm not sure what is your point. Do you think that saying that the "energy is released in the form of heat and radiation" is better? To me that seems like saying "energy is released in the form of heat (i.e. energy transferred) and radiation (i.e. one the transfer mechanisms)". Do you like "heat (some of which is transferred through radiation (some of which is called light))" better?


Two videos I'd like to share (well, one video, and one playlist)...

The first is a 'flashover simulator'. This is a training tool for firefighters to help them recognize the signs of a 'flashover'. A flashover occurs when the radiant heat of the fire is sufficient to cause pyrolysis and ignition of more or less every surface in the room simultaneously (the floor, furniture, etc).

I'm mostly sharing this because sitting in one of these is where it first clicked for me that fire burns gasses, not solids. Watching the fire snake lazily through the smoke overhead, until it felt like I was sitting on the bottom pool, looking up at the surface.

https://www.youtube.com/watch?v=-Z4xxObrTlI&t=40s

The second is Bill Hammack (The Engineer Guy) delivering Faraday's lecture series on The Chemical History of a Candle. This is a truly excellent lecture series, despite its age (they did choose to keep most of the original language, which can take a little getting used to)

https://www.youtube.com/playlist?list=PL0INsTTU1k2UCpOfRuMDR...


That first video is visually stunning, and also gives a complete noob a good idea of what to expect from fire. It's one thing to have some generalized idea that "fire spreads quickly"; it's quite another to see how a seemingly low-key flame will almost instantly burst into an unmanageable fireball. It's interesting to see that there is a pre-emptive warning to a "flashover" - you can see the edge of the fire, for lack of better terms, "licking" or "curling" along the edge. Of course that visual warning is of little consolation if it comes with less than a couple of seconds of time to react; especially without a fire retardant suit.

Imagine how many lives would be saved if every citizen could experience this kind of training first hand. I think inexperienced individuals are likely to think there is time to gather belongings "while the fire is still scaling". How many of us, upon seeing a fire "slowly" taking over a corner of the room, would believe we have plenty of time to collect our most prized or expensive possessions before running out the door?

Random side thought: I wish we'd come up with a better way of testing evacuation drills. All drills do is teach people that every alarm is fake, and so everyone spends minutes packing up their laptops, grabbing their coats, etc. It's disconcerting to watch people "finishing a block of code", drinking their coffee, or texting on their phone at an office when an alarm goes off. And I've seen that at a company where the executive team planned an unannounced drill 2 months after the scheduled annual drill. As in, nobody knew it was a test. And yet it took 15 minutes to evacuate a small building. facepalm


Flashover is visually stunning, but rare these days. Building codes should in most cases prevent it from occurring, at least in non-residential buildings (inside homes it's another matter, as there is often more combustible material than in office buildings). Even if it does happen, it requires a fire to be going on for quite a while already (pyrolysis isn't instantaneous), so everyone inside that room is either a firefighter or already dead, so I don't think training every citizen to recognise it is of much value. At work I'm one of the people responsible for evacuating employees and our only responsibility is evacuation, calling the fire department and trying to extinguish small fires (e.g. a burning waste basket, however that may happen). Anything leading to a flashover is something we should never get close to.

As for the drills, you should make it clear to everyone that getting outside (or behind the nearest fireproof door) is their top priority. Hallways are going to be impassable (without passing out at least) after about three minutes, so 15 is definitely too long. I'd probably try to talk to each one of those people you're describing and make sure they understand what they're doing in that case. While they may not value their life much, their employer is on the line if they die during a fire.


> 15 minutes

I'll be the one leaving as soon as the alarm sounds. I've found this also sets an example and people follow.

My school caught fire when I was 12, due to an electrical fault in the basement, and an adjacent building caught fire when I was about 15. University buildings I was in caught fire twice, once late in the evening and once during an exam.

None of these were serious enough that we couldn't reenter the building within several hours, at least to retrieve belongings, but there'd have been no advantage to lingering inside.


The drills have their place, but I recall a place where they simply, once a month had a walk through of the procedure while doing it. Supposedly, people can actually remember it, and because there isn't any fire drills when the alarm sounds, they take it seriously.


> Random side thought: I wish we'd come up with a better way of testing evacuation drills.

A company-wide policy that if you're still inside 5 minutes after the drill starts without a good, drill-related reason (e.g. crowded stairwell), you get your performance bonus cut this month, and if you're still inside 15 minutes after the drill starts, that's ground for firing?

I can't think of anything else that would make people take drills seriously.

An example I remember from my high-school times that particularly annoyed me was teachers who didn't want to let the class go because they were having a test, and test is obviously more important than a fire drill...


I liked the pun in the argument: if you're inside the building after 15 minutes, you're fired...


  > it first clicked for me that fire burns gasses, not solids
I guess the materials would have some outgassing caused by the heat, but the flashover usually travels over partially-combusted particulate matter suspended in the air. Hotter fires have less smoke + soot, because they consume more of the fuel before it is carried away by convection. You can easily replicate this experimentally: light a candle, extinguish it, then light it again by applying a flame to the smoke emitted from the wick. [0]

Certainly for a standard in-home fireplace fire, most of the outgassing from the fuel is actually water vapour.

[0]: https://youtu.be/C5eTn5d0cvg


The fire moving through the smoke is a "rollover". A flashover occurs when the temperature in a space rises rapidly, igniting most objects in the room, more or less simultaneously.

A rollover is often the trigger for a flashover.


Thanks for clarifying. I got carried away talking about smoke…


Wow – that makes a lot of sense and now I just learned an amazing trick. Thanks!


> fire burns gasses, not solids

That depends on your definition of "burn" and "solid", really. Some fuels only turn into gas due to partial oxidation, or turn directly into plasma - paper would be a good example. Also, solids may have enough surface area to burn all by themselves - coal is a great example here.


fire burns gasses, not solids

Somewhat relatedly, the fire inside an internal combustion engine:

https://www.youtube.com/watch?v=-XkL9JjHY8Q&t=195s


This is definitely the most detailed description of fire I've ever seen.

Note - from an artistic perspective, and AFAIK, wood (and kerosene, and other sooty fuels) produce a "deep"-looking flame, while propane (and other 'clean' fuels) produce a "shallow"-looking flame, due to the black colors introduced by the soot.

Compare the look of the fire in these propane effects: http://www.effectspecialist.com/special_effects_Propane_flam...

to the look of the fire in this kerosene effect: http://mentalfloss.com/article/71663/watch-mesmerizing-fire-...

This is also (AFAIK) why you generally don't see "sooty" colored flame effects - the amount of the coloring agent that is needed to overcome the "darkening" of the soot is impractical, at least for large-scale effects.

Source: That one time I got really excited the talked to people who'd made a (gigantic) flame tornado art piece.


The differences in the examples you provided have more to do with the exposure settings of the camera than the carbon (soot) content of the flame. It's actually the heated carbon particles that give flames their yellow color. The soot itself is what's glowing.


Hmm. I mean, your first point is straight up wrong - I've spent far too much time looking at (and taking pictures of!) different kinds of fire to think the difference in appearance between propane and kerosene is just the camera settings.

But your second point seems pretty reasonable; I'm still not sold. Can you go into more detail as to how the presence of glowing soot in one kind of fire - and absence in the other - would create the appearance of solidity (or lack thereof)?

Edit: Sorry, that came out real aggressive. I mean, can you throw some more words at how the glowing soot changes the look of the fire - regardless of the artistic interpretation of the difference?


Sorry, I was referring to the specific examples you provided. While those flames would indeed look different, those differences would be completely overshadowed by the differences in exposure. In one case you have flames that are _completely_ blown out (due to the dark surroundings), and in the Slow Mo Guys video the camera has been set to capture the details of the flame.

There are absolutely differences between various flames burning in various conditions. When it comes to burning hydrocarbons (propane, methane, kerosene, gasoline, etc), that difference has more to do with the fuel/air mixture than it has to do with the specifics fuel. It is a lot easier to get a good fuel/air mixture when burning a gas like Propane. When a pool of liquid is burning, it is almost impossible to get a good mixture of fuel and air (even with a bunch of fans), so it is burning fuel rich, resulting in a lot of unburned fuel intermixed with the flame.

I would suggest Periodic Videos recent video about the bunsen burner. They achieve both sorts of flames you're talking about with a single fuel source (methane).

https://www.youtube.com/watch?v=in9BGz958jg


If you want to provide visual evidence as an argument, you should really have the same camera and lens and exposure settings. Not some random images off a couple websites. Insisting that there is a difference doesn't reinforce your previous statements at all. The images here https://en.wikipedia.org/wiki/Colored_fire all have the same background and setup to them.

To get back on topic: "Glowing soot" or Blackbody radiation emitted by solid particles in a flame, is by pyrotechnic standards, a washed out flame. Smoke as a background is best not burning. Pretty simple math - to get a red flame of 50 lumens to appear as red when background has gone up to 100 lumens requires a 150 lumen flame. This is why we don't use fireworks in the daylight.

Source: Chapter 11 and 21 in fireworks practices and principals https://exploders.info/Files/Library/eng/pyrotechnics/firevo...


> same camera and lens and exposure settings

Well, yeah, that would make a way more effective argument, but I'd have to obtain a a) manual camera, b) sizable propane fire c) sizable kerosene/etc fire, d) b & c having nearly identical brightness, and e) a place and time to do it. These are doable, but not on an internet-debate timeline.

Think about the differences in your experience with propane and wood fired fireplaces. Most people have had at least one experience with both, and those are the only things I can think of where the flames in both are of comparable size/brightness.

> Smoke as a background is best not burning I'm sorry, I don't understand. Are you missing a word, maybe?

> [link] Amazeballs link, thank you! Do you know the name of the fireworks that produce large quantities of specifically fire? I know them as "gas bombs", but I have no idea if that's the correct technical term.


> Think about the differences in your experience with propane and wood fired fireplaces.

Our eyes and brains are very, very good at working at vastly different lighting conditions and presenting the results similarly. That's why they noted that one should use a camera to compare because our eyes are lying to us. (Seriously: Try to guess proper exposure needed for a photo just with your eyes. Even with experience it's hard and you're likely to be off by one or two EV (i.e. a factor or two or four in brightness!)).

However, if you have EXIF info you can probably still compare brightness of flames across photos, as an image's brightness only depends on the actual scene brightness, ISO, aperture and exposure time. And the latter three are merely factors.


Next time you are playing with clean fires, try spreading¹ different kinds of salts above it (sodium, magnesium and iron salts will be easy to find, cooper is great).

1 - The easiest is to spread them over a metal mesh, try not too mix the salts too much.


That was awesome. Reminded me of the Simpsons episode where the PTA disbanded and Professor Frink teaches a nursery school class: playing with a baby toy, he tells the children they can't play with it because they won't appreciate it on as many levels as he does.

I had a quantum mechanics professor explain to us the difference between a classical mechanics fireplace and a quantum one. Because in classical mechanics energy is distributed continuously, it would emit gammas,x-rays,etc and would kill you if you could see it. The quantum fireplace would behave as fireplaces do, a much less destructive manner. Fun read.


Thoroughly entertaining read.

This article does not cover the history of scientific theory around fire, which is quite fascinating too. Scientists in the 17th century theorized (incorrectly, of course) that fire was an intrinsic property of "phlogisticated" materials [0], which could be burnt to release the phlogiston contained in them. The theory prevailed for over 100 years, until Lavoisier proved that a gas (oxygen) must be present for combustion by measuring the increased weight of phosphorus upon burning [1].

[0] Phlogiston theory: https://en.wikipedia.org/wiki/Phlogiston_theory

[1] Oxygen theory: https://en.wikipedia.org/wiki/Antoine_Lavoisier#Oxygen_theor...


I love the sheer inertia that theory showed. Even after we knew burning added mass, many scientists believed that it didn't disprove phlogiston, only prove that phlogiston has negative mass.


It should be noted that, for a time, the phlogiston theory survived even after Lavoisier's experiments. It only showed that phlogiston had negative weight, what's wrong with that? :-)


I just made the connection between phlogiston and the "magic smoke" theory of electrical devices :)


I thought this would be a link to a Richard Feynman video where he 'explains' fire. But it isn't, so here you go: https://www.youtube.com/watch?v=N1pIYI5JQLE


Dang you beat me to it.


My "What is flame?" - a contest entry: http://www.clarifyscience.info/part/L1Uko

Skimming OP, one thought. My very fuzzy understanding is that, while flame has a spectra shaped like a blackbody curve, it isn't one. Instead, that's the soot emission spectra (due to its diverse mess of bonds). The BBR spectra is much redder and dimmer. This misconception appears widespread.

Meta-observation: Creating fun and correct science education stories is hard, requiring multi-year collaborative effort from many people. It seems a pity there's nowhere on the web to build such.

... Wikipedia has "no original research". The US national science education wiki was/is? just a search engine for high-barrier low-quality paywalled science education magazines. Here in the wretched past, the best we manage to do is accumulate scattered blog and discussion threads. Disrupt, please?


> ... Wikipedia has "no original research".

But they do have a "List of common misconceptions" page.


It could also mention that fire can be considered plasma[1].

1. http://www.askamathematician.com/2013/05/q-is-fire-a-plasma-...


> Combustion is in some sense the opposite of photosynthesis, an endothermic reaction which takes in light, water, and carbon dioxide and produces hydrocarbons.

This is a neat idea that I hadn't heard before!


I've heard it said that Buckminster Fuller described fire as "sunlight unwinding from tree trunks", which struck me as a very poetic way to phrase it.


Quite beautiful. Thank you for the quote.


That's interesting, I've always described metabolism and combustion as the opposite of photosynthesis. You even get back the water that the plant consumed, in the form of steam or condensation dripping out the car's tailpipe, or desert rodents that don't need to drink liquid water because they're so good at keeping the water they metabolize from food!

We just use the atmosphere as a temporary place to store the oxygen, and keep it diluted with enough nitrogen that things don't immediately unphotosynthesize themselves. ;)


Feynmann had some really nice description of this:

https://www.youtube.com/watch?v=N1pIYI5JQLE


The other interesting aspect is that it's more or less the same overall reaction that we perform in our cells to derive energy from organic materials (though the mechanism is very different). I think this is partly why we say things like "exercise burns fat".



Love the way he sort of laughs at these ideas. And his dialect. Sounds like a friendly mobster.


Catastrophe, it's just a way of lookin' at it.


I came here for the Little Mermaid reference and definitely got some answers. Thank you for writing this!


I'm disappointed in whoever effaced the Little Mermaid reference from the title of this article. I thought we were supposed to use the titles the source article used. :b


For the uninitiated[0][1]:

And ready to know what the people know / Ask 'em my questions / And get some answers / What's a fire and why does it / What's the word? burn?

[0]https://play.google.com/music/preview/Tgiolwxoj2rmb6sqjvlnph... [1]https://youtu.be/SXKlJuO07eM


This reminds me of growing up in a really conservative, religious household (think of it as a typical Bible Belt upbringing, except that instead of the south-east part of the US, I grew up in Canada, of Dutch heritage). God created the universe 6,000 years ago, etc. Anyway, to the detriment of their desire for me to grow up believing the same things they did, my parents were big readers and really encouraged me to read, and as a budding nerd, mainly what I read was science fiction and science non-fiction, which led me to have intense arguments with my parents.

I vividly remember my father saying to me at one point, as his response to me making some point about the validity of modern science, that "no one knows how fire works", and that therefore science really wasn't so knowledgeable after all. I remember thinking, "surely if they've figured out atomic fusion, they know how fire works", but I really didn't have a proper response at the time. I had no idea how fire worked. It still seems magical!

Some 30 years later, I finally have the answer...perhaps I should send it to him.


> The heat produced by combustion can be used to fuel more combustion, and when that happens enough that no additional energy needs to be added to sustain combustion, you’ve got a fire.

Sure, but what does "sustain combustion" mean? Well, it provides activation energy for reactions. And here's the thing: lack of activation energy is all that keeps us and other combustible stuff from burning.


This post is a good reminder that innovations are so often made by people with zero or little education.

Scientists don't agree on when humans learned to control fire. Some say 200,000 years ago, some almost 2 million years ago. But clearly, man learned to control and to make use of fire for heating and cooking, for agriculture and as weapons and even for machines long before anyone understood what it is or why it behaves like it does.


I remember when I was about 10 I asked my teacher this question. He had always been a good teacher, encouraging open-minded debate, so I was quite shocked when he told me not to ask stupid questions. (In retrospect, he'd probably just had his fill of me asking unanswerable questions that day.)

Glad to finally get a detailed answer, 39 years later!


If this was written by someone who "isn't a physicist", this is an excellent summary of statistical physics.


He's a pure math guy.


Particles is the wrong word to use when trying to explain radiation heat of fire. Particles usually mean subatomic particles. Particles in general sense are too vague. Molecules would be better.


"Particle" is used at all scales, most usually subatomic, but also microscopic and macroscopic. A car can be viewed as a particle, so I'm not sure it's so wrong to label a group a molecules as "particles".

Also, the author was only referring to particles when trying to explain the concept of thermal radiation where he is actually speaking of subatomic particles, not of molecules.


I expected to be Feyn-mannered.

TL,DR; Stored chemical energy can be released, by adding to its jiggling- and produces enough energy to cause a jiggling chain reaction to other storages nearby.


  > Thermal radiation is produced by the motion of charged particles:
Agreed.

  > anything at positive temperature consists of charged particles moving around
This does not follow: infrared radiation != temperature. Neutron stars have positive temperature…


Dear downvoters, please comment to let me know what I should improve. Thanks, falsedan


You are being downvoted because you appear to be trying to correct a mistake that does not exist in the source material.

I can make sense of your attempted correction by reading "thermal radiation" as "infrared radiation," but those terms don't mean the same thing. Thermal radiation refers to radiation emitted due to a body's temperature, but the peak emission frequency can go up into the visible and beyond if the body is hot enough. (Like the sun and larger stars.) Examples of non-thermal radiation would be e.g. excited state transitions in molecules that produce light in fireflies or the blue light from the lower portion of a candle flame. (The blue part of a candle flame is hot relative to the surrounding air and liquid wax but not hot enough that its thermal emissions would peak in the blue.) Fluorescent and LED light sources also emit visible light by non-thermal processes.


I think the error does exist; infrared radiation is a type of electromagnetic radiation and relies too n charged particles (electrons) absorbing energy, going through various excitation levels, and then emitting photons with particular frequencies.

The error in the source is claiming that charged particles are necessary for a body to have a positive temperature (i.e. what I quoted). The error in the downvoting is a kneejerk correction without explaining + not reading in good faith.


If you had left out "infrared radiation != temperature" it would have been clear. I didn't understand why you said "infrared radiation != temperature" when the source material did not say or imply "infrared radiation == temperature" and I incorrectly guessed at the point you were trying to make.

I was not one of the downvoters, btw.


Thanks for responding and clarifying, it's helpful.




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