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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?