Eli5 Why does fire burn in a visual spectrum for humans


Basically the above. Building a great fire and the thought came up.

In: 2

Fire emits a lot of energy in form of photons, accualy most of them are infrared – > heat but small portion of them are in visible spectrum and some in ultra violet, our eyes see just a small part of all of emission from fire so it’s actually the other way around – burning things emit wide spectrum but we only see small part in visible spectrum

Eyesight evolved as a way of perceiving relevant immediate threats within the relevant spectrum of light. Since fire is a relevant threat, our evolutionary ancestors who could perceive it many hundreds of millions of years ago were the ones to continue their lineage. If other immediate physical threats reflected light outside of our visible spectrum, the surviving evolutionary ancestors whose eyesight was their predominant characteristic of survival would have been capable of perceiving that light. UV radiation was not so important to perceive for survival because, 1: our ozone was still intact during that stage of evolution; and 2: its effects take longer to affect survivability than it takes to pass on our genes to the next generation.

It may have been an evolutionary adaptation. Any of our ancestors who were unable to see fire were undoubtedly less likely to survive.

Because the combustion produces particles in the flame that are warm enough to emit visible light.

All object warmer than absolute zero emits thermal radiation. The amount and frequency spectrum depends on the temperate. We can see the infrared light our own bodies emit but we have instruments taht cant, they are usually called thermal cameras.

When the object reaches a temperature of around 480 °C (896 °F) we star to see a faint red glow. Even if an object gets a lot warmer the still will emit visible light. https://en.wikipedia.org/wiki/Thermal_radiation#Subjective_color_to_the_eye_of_a_black_body_thermal_radiator

The arc produced in welding can be warmer than the surface of the sun, They will contain UV light that can damage you eyes so you should not look at to warm objects

You can detect thermal radiation when IR light hir your skin its heat is up and you can feel that even if objects do not emit visible light. That it is not just warm air you feel is easy to show. But something in-between like a sheet of tinfoil that reflect the radiation. As soon as it is removed you can feel the heat. Hot air does not move that fast.

It is not just as simple as temperature. Gases are very bad at absorbing light taht is why we can see through the atmosphere. It also means they are bad at emitting light so even if they are warm they are quite hard to see. What you see in a wood fire is glowing hot solid carbon. If the wood does not burn up you can clearly see the black carbon that remains, it absorbed light very well which is why it is black. That also mean is is very good at emitting light when it is warm. In the smoke, it is soot particles of carbon that you see.

This explains why some flames for example flames from burning methanol are very hard to see, they contain very little soot and can be practical invisible.

Not all fires do burn in the visual spectrum. Some time ago formula cars ran on methanol, which burns clear. Back then the cars were allowed to refuel at pitstops, and sometimes things didn’t go perfectly. Enough people burned to death in invisible fires that they developed a very overt dance to indicate to the rest of the pit “I am on fire, please come put me out at your earliest convenience.”

Youtube has lots of footage of people dancing the dance and then getting dowsed by support staff.