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We know that fire results from the combustion of organic material and oxygen, but don't you still sometimes wonder what fire is?
我们都知道火是由有机物和氧气的燃烧引起的，但你们有没有想过火是什么？
Why are gas flames blue and wood-fire orange? And why do flames move in such a mesmerizing way?
为什么煤气是蓝色的，而烧柴的火是橘红色的？为什么火焰总是晃动飘渺？
Chemistry may tell us the recipe for combustion, but the light show is all physics.
化学能解释燃烧的实质，但放光放热却是物理现象。
When a flame burns cleanly (like a gas flame, blowtorch or the base of a candle),
当物质充分燃烧时(如煤气、焊枪或蜡烛的底部)，
the heat excites the molecules to release light (usually pale-blue) from atomic transitions. That's quantum mechanics!
热量激发电子跃迁放出淡蓝色的光，这就是量子力学！
Now when the fuel isn't as pure and doesn't entirely burn (like a wood or coal fire or the top of a candle flame),
当燃料不纯且没有完全烧尽（如木头或煤火或蜡烛火焰的尖尖部分），
there's still some blue light, but you don't see it because it's overpowered by light from all the particles of soot and smoke - they're glowing red-hot!
仍然会有部分蓝火，但你看不到，因为烟把它给遮住了，你只能看到红色火焰！
So why do hot objects glow?
所以为什么热的物体会发光？
A process called 'blackbody radiation' makes all objects glow with light of a color depending on their temperature.
一种被称为‘黑体辐射’的过程使得所有物体能够发光，而光的颜色取决于它们的温度。
The reason that you don't see your friends glowing, though, is because we're too cool to glow with visible light - we glow in infrared.
但你的朋友不能发光的原因在于，人体温度太低不能辐射可见光——我们辐射红外线！
But lava, a hot piece of iron, or soot in a flame are all hot enough to glow with that familiar red-orange light.
但是熔岩、一块热铁，或者火焰中的煤烟，都热得足以发出我们所熟悉的橙红色光芒。
And the reason flames are shaped like tongues snaking skywards? Gravity!
火焰飘忽的原因？是因为重力！
The earth's pull is what makes hot air rise, and this convection shapes flames into their familiar form.
地球的引力使热空气上升，这种对流使火焰形成它们熟悉的形式。
If you light a match in zero-gravity, the flame spreads outwards like a balloon - there's nothing to tell it which way to go, so it goes in all directions!
如果你在零重力环境下划亮一根火柴，火焰会是球形——没有重力的影响，火焰会向各个方向延伸！
