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This question already has an answer here:

recently I was working with a thermographic camera and it physical backgrounds.

As far as I understood the thermal radiation is not visible for a human eye (e.g. you can't tell if a coffee is warm or cold from looking at it [ignoring steam that is may present])

But if something gets really high temperature it starts glowing (e.g. some kind of metal).

Why can a human see this glowing? So why is a decent heat not visible while a lot heat is visible?

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marked as duplicate by Floris, Jon Custer, sammy gerbil, M. Enns, Emilio Pisanty Aug 2 '17 at 22:22

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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All "warm" bodies emit electromagnetic radiation, in spectrums similar to the black body radiation. At room temperature, too little of them is in the visible range.

We evolved to have our visible range cover the peak of the sun's radiation, but we do not have many objects close to the sun's temperature around us.

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This can be explained by black body radiation. Any object of a temperature that is above $0K$ (absolute zero) radiates heat and at low temperatures such as with coffee it gives out Infra-red radiation, which is not visible to the human eye. At higher and higher temperatures the intensity of this radiation increases and the frequency of the light goes into the visible spectrum. This means it is visible to us, starting off at around $798K$, the Draper point, where it will glow a dull red. Although this isn't a black body, it is a good approximation of it.

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  • $\begingroup$ This is essentially the right answer, but I'll just point out that all blackbodies emit at all wavelengths. So your coffee cup is emitting in the visible spectrum, it's just far too faint for us to see it. The Draper point is a combination of the shifting of the peak of the blackbody towards the visible and the sensitivity of the human eye. $\endgroup$ – levitopher Aug 2 '17 at 17:05
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Below are plots of the black body spectrum for two different temperature regions. The top plot is for temperature around 0 degrees C to 200 degrees C

This is for temperatures around 800 degrees C to about 3000 degrees C

Notice the large scale change of the x-axis. The wavelength of the visible spectrum is indicated on the far left x-axis in both plots. You can see from this how the peak shifts for varying temperature. Very little visible light is emitted until temperature approaches about 2,500 K. The numbers in the boxes on right are temperatures in Kelvins. 273 K is freezing temperature of water. 373 K is boiling temperature of water. Close to very hot coffee.

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