In Feynman's terms temperature is the speed at which atoms are 'jiggling'. Now, let's suppose I've just eaten a sizable dinner, and my body temperature just got a tad up. Am I emitting more photons in the infrared spectrum, or are my atoms 'jiggling' faster and bouncing more of the air component atoms (nitrogen, oxygen etc.)?
Or is it the same, and all heat energy transfer is mediated by photons?
Surely not all heat is mediated by the IR spectrum of the light? Otherwise only objects made of materials able to absorb IR photons energy levels would heat up?
Can't quite decide if it fits the scope of my question, but: where does heat reaching my skin from Sun come from? I plan to politely ignore answers 'from the Sun' 🙂 I mean does it come from IR radiation?
And more generally: why is heat commonly attributed to IR spectrum? I take heat is a form of energy, if so, why doesn't my yellow car heat me up 'by it's photons' – visible spectrum is supposed to be of higher frequency = energy than IR?
Best Answer
When atoms jiggle, their charged constituens get accelerated, and accelerated charges radiate electromagnetically (keep in mind that like most of physics, this is a lie-to-children).
The (idealized) distribution of electromagnetic radiation is given by Planck's law, which cannot be derived from the classical picture I gave above because of the so-called ultraviolet catastrophe - you need quantum mechanics for that.
The infrared part of the spectrum has no special association with heat energy - it's just that radiation intensity will peak there at 'moderate' temperatures and in particular room temperature. For example, the sun has an effective temperature of ~5800°C and peaks in the visible spectrum, whereas the cosmic microwave background corresponds to thermal radiation at ~2.7 K.
As to your other question - it is indeed possible to transfer heat via thermal radiation as the emission processes come with associated absorption processes. The other modes of heat transfer are convection (ie moving heated matter around) and conduction (ie the atoms transfer their jiggling by collisions).