How does fire emit light (the sun, a candle, a fire, etc.)?


How does fire emit light (the sun, a candle, a fire, etc.)?

In: Physics

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.

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.

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.

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.

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.