Eli5: If heat is molecular vibration speed, how is seeing in infrared “heat vision”?



Does the color in infrared always correspond to temperature? Like, more so than it does in the visible spectrum?

In: Physics

All objects glow, in a “color” that depends on their temperature. You see this in hot metal, which glows dull red, then bright red, then orange, yellow, and brilliant white as you heat it up (eventually, it would approach a light blue if it kept getting hotter).

At room temperature, this glow is basically zero in the wavelengths our eyes can see. But it’s quite bright in the infrared. So what we mean by “heat vision” is “vision that sees the glow of things that are warm by human standards”.

Its heat vision because it can be used to view the electromagnetic radiation emitting from people’s bodies which corresponds to body heat. IR corresponds to heat just as much as the the visible spectrum. Its just named for the use most people associate with it.

IR exists in remote, if that were more popular it would be called remote rays instead

Molecules vibrating causes their electrons to vibrate, which produces electromagnetic radiation. More vibration produces higher frequencies of radiation. Enough vibration, and you see visible light, starting as a red glow. However, at room temperature, this frequency is in the far infrared.

> Does the color in infrared always correspond to temperature? Like, more so than it does in the visible spectrum?

The color of the radiation corresponds to its temperature totally irrespective of where the human visual spectrum begins and ends.

I think the question reflects a slight misunderstanding about what color is. The color of a given light (or really, it’s wavelength/frequency) is precisely what *makes* it part of the visible spectrum or not. If you go further up the frequency scale you get additional “colors,” ultraviolet, that are outside of human perception, and if you go further down, you get infrared “colors”.

The hotter something is, the higher frequency of radiation it glows in. Things which are warm, the kinds of temperatures that human bodies and habitats are, glow in the infrared. Things which are very hot, start to glow in the visible spectrum. When they’re super super hot, you get ultraviolet and higher frequency radiation. That’s really the only rule that physics cares about here, hotter = higher frequency/shorter wavelength. Visible/invisible doesn’t really come into it.

TL;DR: Seeing in regular visible light is also ‘heat vision’, it’s just that thermal radiation in this band only comes from much hotter things like lightbulb filaments and the sun.

(note: I’m using “color” as basically interchangeable with “wavelength” here; whether it’s really correct to call UV and IR light frequencies “colors” is kinda a question of perceptual philosophy.)

Everything that’s not at absolute zero emits some thermal radiation. The exact spectrum of radiation is governed by the temperature. The jiggling of the atoms relative to each other literally does this! In simple terms the charge of different parts of atoms oscillates as the atoms move about which produces oscillating electric and magnetic fields – I.e electromagnetic radiation. Technically everything emits all frequencies, but the amount emitted becomes vanishingly small at most frequencies.

Very hot things (the sun) emit thermal radiation from microwaves, to infrared, through visible light and into the higher frequency ranges too (UV).

Things in the human living range emit practically zero visible light, but perfectly reasonable amounts of infrared radiation. This makes infrared useful in determining the temperature of “cooler” things.

There isn’t any real difference between visible light and infrared, it’s a continuous thing. Logically evolution has made our eyes most sensitive to light that’s smack bang in the middle of the spectral peak from the sun. It takes an object of several hundred degrees to glow visibly, so there are plenty of hot things that don’t glow visibly that we think of as hot.