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What Gives a Flame Its Color?

The orange-yellow flame of a campfire may bring back memories of singing with family and friends and enjoying food that's too charred to actually taste good. But have you ever noticed that all campfire flames appear very close to the same color?

Though different from campfires, flames from butane lighters also appear very similar in color, no matter what lighter brand you have. And propane flames, whether from your grill or a blowtorch, are bluer than butane flames and campfires.

Why are flame colors unique to different kinds of fuel? In today's challenge, you will answer this burning question.

You may recall from this challenge that a flame is a chemical reaction between fuel and oxygen. As a fuel combusts, it releases energy that is carried away as heat. Gases produced in the reaction and small, hot pieces of unspent fuel are flung turbulently upwards from the reaction site.

Many flames look like they have two parts, which have two different colors. These colors represent the two fundamental ways hot matter emits light: black body emission and molecular emission. In today's challenge, we focus on the latter.

Molecular emission is light that's given off during molecular transitions in the gas molecules produced by combustion. More specifically, electrons in these molecules emit light as they lose energy.

Subatomic particles like electrons absorb and emit energy in a very different way from how your body, your car, or most other objects that are much larger than a molecule do. That's because they follow the rules of quantum mechanics.

To help us make sense of this difference, let's imagine an electron as someone at a pool who's presented with a series of diving boards that all have different heights. The diving boards represent the excited states of the gas molecules produced by combustion.

In an excited state, a molecule's electrons have a definite amount of energy. A molecule has a set of these states, and can only enter one of them if it absorbs the precise amount of energy associated with the state.

Analogously, our electron at the pool can only climb to the top of a particular diving board by gaining a particular amount of energy from the fire.

So what does this picture have to do with the light produced by a flame? What goes up, must come down.

After the flame boosts our electron divers to the top of a diving board, they release the energy in a splash when they hit the pool. The higher the board, the bigger the splash. But in this quantum world, where diving boards have specific heights, all the splashes we see will have one of a few particular sizes.

Likewise, in a flame, gas molecules readily absorb the heat released by the combustion reaction. Electrons climb the ladder of excited states, but they soon dive back down to lower-energy states, releasing energy in the form of light.

It turns out that the energy carried by light is what determines the color that humans perceive when it enters the eye. The various colors of light in a flame are the "splashes" that occur as electrons fall out of different excited states. Every fuel produces different gas molecules during combustion, which emit a particular color combination thanks to their unique set of excited states.

Today's Challenge

A flame's spectrum shows the amount of light emitted, separated by color (usually expressed as wavelength \(\lambda).\)

Which spectrum best represents the part of the flame where molecular emission is taking place?

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