There is a great discussion on infrared heat and thermal radiation from the subreddit /r/askscience that I would like to discuss.
The question was: Is the heat I feel when I face a bonfire transmitted to me mostly by infrared radiation or by heated air?
Question: “When I face a bonfire from about 15 or so feet away, the skin on my face feels hot. When I turn and face away from the fire, the skin on my face feels much cooler. I’m guessing that if the heat I felt came from the heated air around me, then it wouldn’t really matter which way I faced if I were just rotating around a point. Does my skin heat up mostly because of the (I’m guessing infrared) radiation coming from the fire?”
Answer: “Yes, you are right.
The thermal radiation created by the bonfire travels away in all directions. Heat that is transferred via convection mostly travels upwards as the heated air billows up. If you are to the side of the fire, the heat you receive is transferred via thermal radiation (i.e. infrared light). If you are standing directly above the fire, you receive heat from both thermal radiation and convection. For this reason, directly above the fire is the hottest place to be. I don’t recommend it!
Note that thermal radiation can include many different wavelengths of electromagnetic radiation and not just infrared, although infrared is the dominant type near room temperature.
For a bonfire, the thermal radiation is composed of both infrared radiation and visible light in significant amounts.
UPDATE: In loose usage, the term “thermal radiation” means “radiation that is able to heat an object upon being absorbed by the object”. In this usage, all electromagnetic radiation is thermal radiation, from radio waves to gamma waves. In the more strict usage of the term, “thermal radiation” means “radiation that is produced in a broad spectrum that depends on the temperature of the source”. In this stricter usage, the visible light from LED flashlights is not thermal radiation, since LED flashlights do not operate that way. Each photon from the LED flashlight is not different from a photon of the same frequency from a campfire – they can both heat something they strike. But the spectral frequency distribution of the photons from an LED bulb is not thermal.”
Therefore, the reason thermal radiation produces heat is because it is specifically operating in the range of the infrared spectrum of light.
When it has the right wavelength, it will produce heat. So the answer to the person’s question of whether they are feeling heated air or thermal radiation (infrared heat) is that when you stand next to a bonfire, you are feeling infrared heat!