There are portions of the electromagnetic spectrum that the human eye can’t see. This includes thermal infrared radiation. This is often radiation with long wavelengths. Almost everything emits thermal radiation - even humans. Objects like a stove element on high, emit radiation from the visible spectrum when it glows ‘purple’ https://josueoeyjq.theblogfairy.com/29919758/the-fact-about-hand-held-thermal-imagers-that-no-one-is-suggesting