Fire is a chemical reaction that produces light and heat. The warmth felt from a safe distance, even without physical contact with the flames, confirms that fire emits infrared light. This radiant energy is responsible for the intense, immediate heating we experience when facing a flame. Understanding this phenomenon requires examining what infrared light is and how the combustion process generates this invisible form of radiation.
Understanding Infrared Radiation
Infrared (IR) radiation is a type of electromagnetic energy that exists just beyond the red end of the visible light spectrum. IR waves possess longer wavelengths than visible light, typically ranging from 780 nanometers up to one millimeter. Since the human eye is only sensitive to wavelengths between 400 and 700 nanometers, IR light remains invisible to us.
All objects with a temperature above absolute zero emit thermal radiation, most of which falls into the IR spectrum. IR is categorized into near, mid, and far infrared, based on its wavelength, which dictates how the energy interacts with matter. The far-infrared range is commonly referred to as thermal infrared.
The Mechanism of Thermal Emission
Fire produces light and heat through combustion, a rapid chemical reaction that releases stored energy. Fuel combines with oxygen, resulting in extremely high temperatures, often ranging from 600 to over 1,000 degrees Celsius for typical wood fires. This intense heat causes materials within the flame, such as gases and tiny solid particles of soot, to glow.
This glowing is thermal radiation, fundamentally linked to the temperature of the emitting object. Energy released by vibrating charged particles within the hot materials is converted into electromagnetic waves. The specific range of wavelengths emitted is governed by the object’s temperature, a principle simplified by the concept of blackbody radiation.
Since fire temperatures are relatively low compared to the sun, the peak of their energy emission spectrum falls heavily into the infrared range. While the hottest parts of the flame emit shorter wavelengths that produce visible orange and yellow light, the vast majority of the total energy is released as invisible infrared radiation. Fire is visually bright, but its energy output is dominated by IR light.
Why We Feel the Heat
Infrared radiation is strongly associated with warmth because it efficiently transfers energy to matter, particularly human skin. Unlike heat transferred by convection, which relies on the movement of hot air, IR radiation travels directly through space as an electromagnetic wave. When IR photons strike the skin, their energy is absorbed by molecules, causing them to vibrate more rapidly.
This increased molecular vibration translates directly to a rise in temperature, registered by our thermal sensory nerves as heat. The IR wavelengths emitted by typical fires are readily absorbed by water molecules present in skin tissue. This direct energy transfer explains why the sensation of warmth vanishes instantly when you turn your back to a fire, blocking the radiation path.
This property of thermal radiation is the basis for technologies like thermal imaging cameras. By sensing the infrared energy emitted by all objects, these cameras create a visual map of temperature distribution, making the heat signature of a flame visible even through smoke.