I recently got asked a question by one of our customers that I found to be very fitting for this time of year: Is the heat I feel when I sit close to a fire from infrared radiation or heated air?
Question: Hi Chris, Thanks for all your help explaining infrared light to me. However, I do have one more question. When I get close to my wood stove/fireplace I can feel heat from about seven or so feet away. The skin on my face feels hot and the exterior of my clothes start to warm. When I turn around and face away from the fireplace, the skin on my face starts to cool. I’m going to guess that if the heat I feel comes from heated air around me, then it wouldn’t matter if I faced the fire or not. So my question is, does my skin heat up because of the infrared light/heat coming from the fire?
Answer: Yes, you’re right.
The thermal light radiation (or heat) that’s generated by your wood stove travels away in all directions. Now, remember that there is three types of heat: convection (heating through the air, like an oven); conduction (heating through touch, like an electric blanket); and thermal radiation (heating through infrared light which is absorbed and stored by an object). Heat that is transferred by way of convection travels upward (hot air rises) as the heated air billows up. However, if you’re sitting to the side of the fire, the heat you feel is received via thermal radiation. If you’re standing right above the fire, you’ll both receive heat from thermal radiation and convection. For this reason, directly above your fireplace is the hottest place to be. This is why when cooking food over a fire it is placed directly above the flame.
It’s important to note that thermal radiation includes different wavelengths of electromagnetic radiation (radiation including visible light, gamma rays, X-rays and radio waves) not just infrared. However, infrared light is the most abundant type near room temperature and above (when you feel heat you are most likely feeling it from a source, not just the air; therefore, when you feel heat, you feel infrared). For a bonfire, fireplace or wood stove, the thermal radiation is composed of both visible light and infrared radiation.
Update: In loose terms, “thermal radiation” means radiation that’s able to heat an object after being absorbed by it. This means, all electromagnetic radiation (radiation including visible light, gamma rays, X-rays and radio waves) is thermal radiation. In more strict usage, thermal radiation is radiation that’s produced in a broad spectrum which depends on the temperature of the object. In the strictest of usage, the visible light from LED bulbs or flashlights is not actually thermal radiation, since LED lights don’t operate that way. Each photon (or light particle) from the LED is not different from a photon from a campfire–they can both heat an object they strike. But the spectral frequency of the photons from an LED lightbulb is not thermal and as a result does not produce its own heat source.