# Glowing metal: different visual spectrum? [duplicate]

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?

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

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.