How does fire produce light? How does anything hot produce light? It might be simple but I have no clue…
In: Physics
Light in general is produced by charged particles jiggling. Electrons are way lighter than protons, and those are the only two common charged particles, so electrons are easier to jiggle. In hot objects, the electrons get jiggled by particles colliding into each other. Enough heat generates sufficiently fast jiggling to make visible light. This is how fire and incandescent bulbs make light, as well as the sun for the most part. Other methods include using clever chemistry and high voltage to jiggle electrons. These two methods both tend to generate very precise frequencies of light, and are used in LEDs and fluorescent lights respectively.
The term for hot objects production of light is “black body radiation”
Anything above absolute zero will radiate some energy in the form of electromagnetic radiation. For very cold objects, it is very long wavelength and low intensity, barely detectable by even instruments. Things do this because of entropy, energy always seeks to be at the lowest energy level possible.
As atoms get hotter, the energy they release gets more intense and of a shorter wavelength. At a certain temperature (starting around 400C) that wavelength starts to get into the visible spectrum and we see it as light.
Light is an electromagnetic wave, and electromagnetic waves are produced by the movement of any charged particle.
When things get hot, the atoms that make them up vibrate. Since atoms are made of charged particles (electrons and nuclei), when they vibrate they produce electromagnetic waves, i.e. light.
Light is jiggling of space, when atoms jiggle, they slightly jiggle space, so when things are very, very hot, they jiggle a lot, making space jiggle enough for us to see the jiggle.
When something burns, connections between atoms are being released from the bouncing of other atoms around it, and what are the connections? they are forces, and it takes force to break bonds of forces, and that will make space jiggle enough for you to see it.
Light is converted from molecular motion when there is no other matter to pass the energy onto. Heat results from molecular motion.
Heat is crazy random molecular/atomic motion (rotation, vibration, and flying through space).
Light is an oscillating electromagnetic wave. Literally oscillating from positive to negative charge across space. (Google an electromagnetic wave for a reasonable diagram)
Atoms and molecules are made up of electrons and protons which are negative and positively charged. When they jiggle/vibrate (and accelerate) the oppositely charged particles create an electromagnetic field between each other.
Since light is just an electromagnetic wave moving through space you can sort of grasp how when the field is generated from oppositely charged particle motion it spreads away from it. Thus light is created.
Light MUST be generated from a dipole (charges accelerating towards/away from each other). This is the same mechanism for light absorption. A dipole is required. Not only that but light must hit the dipole perpendicularly for absorption to happen. Light waves parallel to dipoles do not couple.
Everything is emitting light at all times. The hotter something is, the shorter the wavelengths of light that it emits.
We can only see a very narrow range of wavelengths of light. Most things, most of the time, emit light at wavelengths much too long for us to see. As things heat up, the wavelengths they emit get shorter until they are emitting light we can see, starting with red since that is the longest wavelength we can see and moving up the spectrum towards blue as the temperature increases and the wavelengths get shorter.
This is how thermal imaging works: It detects wavelengths of light that are a little longer than we can see normally in order to “see” the glow of objects that are too cold to emit visible light but are still glowing at different wavelengths based on temperature.
Fire is a chemical reaction, combustion. Both fires and hot things have a lot of energy, so much so they radiate the energy around them, exothermic.
Now when it comes to light, or electromagnetic radiation, this comes from the electrons of the atoms of the molecules that are warm or combusting.
Electrons are weird, they only like to be in certain states depending on their energy and the configuration of the atom they are orbiting.
When an atom is warm, the electrons can have so much energy they jump up into a higher energy level. Imagine a giving a child a red bull, the added energy of the sugar and caffeine will cause them to enter their manic configuration, from relaxed to Satan’s spawn.
Now when the electrons relax back to their “resting” configuration they need to lose that energy. They do this by emitting an electromagnetic wave, the energy of which is equal to the difference in the energy levels.
If you divide this energy (E) by the Planck constant (h) you’re left with the wavelength of the wave (λ). So, depending on the energies released you may get blue light, you may get infrared or you may get UV. Due to the varying jumps of electrons between levels, in most cases you’ll get a mix, which is why when something is white hot it’s really high temperature, lots of wavelengths = lots of energy.
Now, you may be wondering why electrons relax back to a lower energy level if it’s still hot and has a lot of energy. Well, electrons are weird. The chemical and physical world doesn’t like to be high energy, just like us.
It’s unfavourable for any particle to be in a high energy state (entropy) and will relax effectively instantly, realise it’s got too much energy in its ground state and jump up to another energy level again. This will happen billions upon billions of times per second for any particle that has too much energy.
So the result, until the energy has fully dissipated, the hot/combusting things will emit radiation of some form.
Note: I’m on mobile so I apologise if this is formatted horribly.
Other note: I have a chemistry degree so this is the most ELI5 I can go without overcomplicating things.
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