How does fire emit light (the sun, a candle, a fire, etc.)?
In: Physics
Imagine you are holding two very soft citrus fruit (i.e. clementine.) You start to shake your hands back and forth uncontrollably and so violently that you start squeezing the fruit hader trying to hold on to it but eventually it’s sweet juices bursts from its thin peel.
In this analogy Your hands are two charged particles and how fast they vibrate when the juice leaves the citrus is the frequency of the elecrto-magnetic spectum where visble light begins
When you feel the heat from fire, you are feeling atoms in the fire shoot off their energy because it’s too much for them to hold. That energy hits your skin and turns into heat.
But, that fire sends out energy of all kinds, some you can see and some you can’t. The energy you can see becomes all of the colors only once it hits your eyes and your brain interprets it. But there are “colors” that our eyes and brains can’t see, too. The closest ones to our sight are called “ultra-violet” and “infrared”. Others include X-Ray, microwave, radio wave, and gamma rays. They are all just forms of energy that wiggle around at different speeds. We use tools to “see” and use the different “wavelengths” of energy.
Ultimately, the universe doesn’t care whether or not fire emits light. Fire emits energy in the form of electromagnetic radiation which likes to wiggle about space to the tune of many different beats. Our brains happen to interpret a few of those beats and show them to us as light.
A lot of energy is put in to make things burn. This energy “excites” the electrons makes them go up another level. As they come back to their “ground state” they release that energy in the form of light.
Light is a wave, like water. Water waves are made when something disturbs the water, like dropping a pebble into the water. If something jostles the the pebble another rippe is sent out.
Light is a wave in something like water, called an electro magnetic field. This is the thing you feel when you try and place to magnets close to each other. Instead of pebbles, whenever an electron is bumped it makes ripples in this field. That ripple is light.
The reason the electrons are getting bounced in a fire is because hot materials have faster moving atoms (that’s what temperature really is, fast or slow atoms). If they collide they bounce the atoms and electrons around, causing ripples (light) to spread out from the impact.
Think of electrons like they are cars moving down the highway, and photons (light) like a signal drivers use when they are switching to a slower lane. The slower cars are on the right, and the faster cars are on the left (in America under ideal driving conditions). As slow cars get more energy, they go faster and switch to the next lane over to their left. They can also lose energy, slowing down in the process, but when they do this, they use their signal and emit light.
Now, think of a light bulb while thinking about these cars. You turn on the switch and electricity moves through the light bulb, giving those cars (electrons) lots of energy. Those cars move over into the left lanes because they are going really fast now, but they don’t want to stay there. So, the cars move back over a lane, using their signal (the electron emits a photon) each time they do this. So long as there is electricity powering that light bulb, these cars (electrons) will keep changing how fast they’re going and using their signal, “creating” light.
Fire is a chemical reaction, it gives off matter (ash, soot, and smoke), heat, and massless photons as byproducts. The photons given off by fire are called incandescence. What you see is based on the photons and the frequency they vibrate. Fire will typically be red and orange which are lower on the spectrum. Stars can be a bit different and often give off photons higher on the spectrum. They can display colors like blue or white which are emitted photons with waves properties vibrating at a higher frequency (and more energy) than your typical fire.
All atoms above absolute zero (think of your freezer at home, it’s even colder than that), jiggle. The warmer things are, the more those atoms jiggle around.
Atoms have a core of protons and neutrons, and a shell of electrons. The core is really small, and the electrons form shells at different distances from the center. The closer it is to the core, the more energy there is holding it in place.
When you jiggle atoms, electrons from one atom are hitting the electrons of other atoms. When they hit hard enough, an electron is bumped to a further shell and the energy holding it in is let go. It’s like releasing a stretched elastic band, or dropping a coin from the top of a bridge. This creates electromagnetic energy. The more you jiggle, the more powerful (shorter wave) it is.
At lower temperatures, atoms close to each other radiate radio waves and infrared. Get hotter, and you get light. Even hotter, UV rays or even X-rays. The hotter the atoms get, the faster and harder they jiggle, and electrons fly more and more.
When you light something on fire, you are making it (or the gas that burns) just the right temperature to show that color. Cooler flames are red or yellow, warmer flames are green or blue, and we know this because blue light has a shorter (higher energy) wavelength than red light.
Tl;dr: Jiggling makes warm stuff make light.
When you put energy into an atom the electrons move farther away from the nucleus, but they want to go back to normal. To do that they need to get rid of that energy. Electrons get rid of that energy as electromagnet radiation. Depending on the strength it comes out as heat, radio signals, or light.
Edit: the light receptors in your eyes response to certain types of these waves which become photons and trigger the nerves to send signals to your brain.
It’s a couple of different things going on, and how much each process contributes to the light produced depends on the type of fire.
The first one, which is responsible for most of the light from things like campfires and candles, is called black body radiation. This is the phenomena that all objects convert heat to light. You don’t see cold objects light up, because the amount of light and the wavelength (color) of the light depends on the temperature of the object. Colder objects throw off low energy light like radio waves and infra red. Heat them up more and they start glowing red. More and you’ll see more yellow, blue, or white light. Fires give off light like this because the gasses and tiny solid particles flying up from the fir are hot enough to glow.
The other process is due to the burning process giving the electrons inside of the fuel energy. Electrons in an atom are on a kind of energy ladder, where they can step up and down rungs. The burning process takes an electron and bumps it up the ladder a step (or maybe two or three), then the electron falls back down to where it was before. To do this it has to release all that energy that it just had, which it releases as light. When light is produced in this way, it’s only in a very narrow wavelength, which means you see a specific, pure color of light from this process, some of which you’re unlikely to see from black body radiation which sort of releases a blend of wavelengths together. This is what you see when you burn copper and the flame is green.
The process called fire is exothermic, it heats the chemicals involved. If this hot gas is hot enough, the electrons in the hot gas can shed energy and collapse into a lower energy orbital by creating a photon. If those photons are in the right frequency range, you can see them as light.
The Sun is not a fire. It’s a completely different, nuclear, process. However, it is also exothermic, and it heats the atoms involved up and they generate light for similar reasons.
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