Ground Truth · everyday physics, made operable

The Two Lights of Fire

A flame is not one thing giving off light one way. It carries two lights from two unrelated pieces of physics, and it is barely ionized at all. Drag the heat and watch a glowing gas climb from invisible infrared to yellow to white; switch to the flame's other light, the sharp blue lines of excited radicals; then run the equation that settles the oldest internet claim about fire, that it is a plasma, and watch it fail by twenty orders of magnitude. Every number here is recomputed in your browser and checked offline first.

Ask what fire is and the honest first answer is a process, not a substance: a self-sustaining chemical chain reaction, fast oxidation, releasing heat quickly enough to keep itself going. The part you see, the flame, is hot gas: combustion products, unburnt fuel, and (in a yellow flame) a fog of tiny soot particles, all glowing. But how it glows turns out to be two separate stories, and telling them apart is most of the answer to why a candle is orange and a gas ring is blue, and why the blue one is the hotter of the two.

1The first light: glowing because it's hot

Anything hot enough radiates light. This is incandescence: thermal radiation, the same physics as a stove element or the Sun. Set a temperature and watch the colour and spectrum a perfect radiator would show. The yellow of a candle is the incandescence of its soot, at roughly 1500 kelvin.

The lesson hides in that visible fraction. At a candle's soot temperature the spectrum peaks near 1700 nm, deep in the infrared, invisible, and only a fraction of one percent of the radiated power lands in the range your eye can see. The warm yellow of a candle is the thin short-wavelength tail of a fire that is mostly pouring out heat you feel but cannot see. Push the slider up: the peak marches leftward toward visible, the colour whitens, and a far larger share of the power finally becomes light. That is why a hotter thing looks whiter. Not a different substance, just more of the curve crossing into the visible.

2The second light: glowing because of what it's made of

The blue base of a gas flame is a different light entirely. It is not a hot glow but emission: specific molecules and atoms, briefly excited by the reaction, dropping back down and releasing photons at sharp, fixed wavelengths. This light carries a fingerprint. Toggle the sources and drop a pinch of a salt into the flame.

The clean flame's own light comes mostly from two excited radicals born in the reaction zone: CH*, with a band head around 431 nm, and C₂*, whose green-and-blue Swan bands sit near 473, 516 and 563 nm. Together they read as blue. This is the crucial thing most explainers get wrong: a blue gas flame is not the blue end of the hot-glow sequence. A gas at flame temperature, put on the blackbody curve of Instrument 1, would show a dull red, never blue. The blue is radical emission, a band spectrum from excited molecules, and it has nothing to do with temperature. Add sodium and the whole flame floods with a single orange line at 589 nm, the same Na D line as an old sodium street lamp, and the reason a boiling pot that spits salt-water flashes yellow. Each element you drop in answers with its own fixed signature.

So why is the blue flame the hotter one? Because blue means clean. A yellow diffusion flame (a candle, a gas ring with the air shut off) makes soot, and that soot incandesces brightly at a modestly hot temperature: light one. Open the air (a Bunsen's collar, a stove's premixed burner) and the fuel burns completely before it can form soot: almost no incandescence, so the flame is dim and you see mainly the faint blue emission, light two, yet the complete burn runs hotter. Brightness and temperature come apart. The luminous flame is the cooler, dirtier one.

3Is fire a plasma? Run the equation.

A plasma is a gas ionized enough that free electrons and ions govern its behaviour. The claim that "fire is the fourth state of matter, plasma" is everywhere. The Saha equation tells you what fraction of a hot gas heat alone can ionize. Slide from a campfire up to a lightning bolt and read it off.

At every temperature a chemical flame can reach (a candle near 1700 K, wood around 1300 K, a roaring blue Bunsen near 2000 K, even an oxy-acetylene torch at 3400 K) the thermally ionized fraction of the air is so far below one part in a billion that calling it a plasma is simply wrong by the definition. Meaningful ionization by heat alone does not arrive until many thousands of kelvin: a welding arc, a lightning channel near 30,000 K, the surface of a star. Those are plasmas. A candle is not.