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ELI5 Yahaaa

About a hundred watts of it

You Are Glowing Right Now

Everything warmer than absolute zero gives off light. Yours is the wrong colour for eyes.

A body at 37°C Plainly visiblein infrared
The glow is not a metaphor. It is light, at a wavelength your eyes do not have receptors for.

  1. Step 1 of 5

    Jiggling charges make light. Everything jiggles.

    1 Heat isatomsmoving2 Atoms carrycharge3 Movingchargemakes light4 So anythingwarm isglowing
    An ice cube glows too. It simply glows less than the room around it

    Heat is atoms moving, atoms carry charge, and moving charge radiates. So anything above absolute zero is throwing out light all the time, without exception. Being warm and glowing are the same fact.

  2. Step 2 of 5

    The temperature picks the colour.

    Room temperatureFar infrared —invisibleYour body, 37°CInfrared —invisible500°CDull red — firstvisible1,500°COrange-white: afilament5,500°CYellow-white,like the Sun
    One ladder, and your eyes only see the top of it.

    Hotter means shorter wavelengths. A cooling iron bar runs through the whole demonstration: invisible at first, then dull red, then orange, then yellow-white. Nothing changes but the temperature, and the colour follows it.

  3. Step 3 of 5

    Human eyes start at red and stop.

    What you emit Peak around 9micrometres —infraredWhat you can see 0.4 to 0.7micrometres — visible

    There is nothing special about the visible range except that we evolved receptors for it. Just past red is infrared, which is exactly where a body at 37°C radiates most strongly. We are glowing squarely in a band we happen to be blind to.

  4. Step 4 of 5

    Plenty of things are not blind to it.

    Snakes withheat-sensingpitsThermalcameras, doingthe same jobAnd nodarkness helpsyou at all

    Pit vipers have organs that sense infrared and can strike accurately in complete darkness. Thermal cameras convert the same signal into false colours. Neither is doing anything exotic; they are simply picking up light we cannot.

    Which is why a thermal camera finds someone at night, through smoke, with no light source of any kind. They are the light source.

  5. Step 5 of 5

    It also means you are constantly losing heat as light.

    You in a roomRadiation leaving you, alwaysRadiation arriving from warm surroundings
    Feeling cold is losing this contest.

    A resting adult sheds roughly a hundred watts, and a sizeable share of that leaves as radiation rather than by touching anything. It is why a room with cold walls feels cold even when the air is warm — you are radiating to the walls and they are not radiating enough back.

    A survival blanket is shiny for exactly this reason: it reflects your own infrared back at you rather than letting it escape.

The short version

Everything above absolute zero radiates light, with the colour set by temperature — your body glows in the infrared, which is invisible to eyes and obvious to a thermal camera.

Try it yourself

Hold your palm near your cheek without touching. You feel warmth arriving across the gap. Some of that is radiation — light travelling from your hand to your face.

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