# What is the difference between radiation, emission and reflection in terms of heat?

I am reading heat transfer and I get confused between these terms. I know they are very basic terms but I can't relate them with heat. Any help is appreciated.

## 3 Answers

Heat IS radiation in the sence that it is infrared electromagnetic radiation.

Emission is how much energy (heat) one radiates outwards due to ones internal energy. Normally this behaviour is modelled as a blackbody radiator with an emissivity and absorbtivity (spelling please).

Reflection is the part of incoming radiation that is not absorbed by the blackbody (unless it is a perfect blackbody) and is thus reflected outwards.

• In my mind heat is not radiation, and radiation is not the net of absorption and emission. I think the lesson of this answer is that the words have different meanings to different people and in different contexts. – garyp Apr 10 '20 at 14:48

Speaking as an architect and engineer I am familiar with three forms of heat emission or transfer:

• Conduction in which thermal motion of particles is transferred from a hot body to a cold one in contact with it.
• Convection in which heat is similarly transferred to a fluid medium, the locally heated volume of which then expands and rises, drawing in more cold fluid.
• Radiation in which electromagnetic energy, primarily in the infrared band, is emitted to carry away the excess energy of the agitated particles.

Thus, from my perspective one can give the more physics-related answer that in this context:

• "Heat" may refer either to the random kinetic energy of individual particles in a mass, or to electromagnetic radiation in the infrared band.
• Radiation is the emission of heat via black body radiation, which one may describe either as a classical wave or a stream of photons, according to which mathematical treatment is the more convenient at the time.
• Emission is the passing of heat energy in any form to somewhere else.
• Reflection is the blocking of incoming thermal radiation and returning it back to the medium in which it arrived.

Think about the duality of wave and particle proposed by Paul Dirac.
Emission is used in context that it is considered as particle, like blackbody emits photon energy (energy in packets). Radiation is used in context that it is considered as wave, like frequency of electromagnetic waves.