Is there a difference between thermal radiation and infrared radiation?

Understanding the Concepts

Yes, there is a meaningful distinction between the terms thermal radiation and infrared radiation. While these terms often overlap in everyday language, they refer to different concepts in physics.

Thermal radiation is electromagnetic radiation emitted by an object due to its temperature, encompassing all frequencies produced by the thermal emission process. This includes not only infrared frequencies but also radio waves, visible light, ultraviolet light, x-rays, and gamma rays. In contrast, infrared radiation refers specifically to electromagnetic radiation with frequencies in the range of $0.3 \, \text{THz}$ to $400 \, \text{THz}$, regardless of the process that generates it.

The Thermal Emission Process

Thermal radiation results from atoms and molecules in an object moving randomly due to thermal energy. As they collide and vibrate, they emit electromagnetic radiation. This process occurs continuously for all objects made of atoms, and to achieve no thermal radiation emission, an object would need to be at absolute zero temperature, which is unattainable due to quantum uncertainty.

The hotter an object becomes, the more thermal radiation it emits and the broader the spectrum of emitted radiation extends, including higher frequencies. For example, an object at room temperature (approximately $294 \, \text{K}$) emits thermal radiation that is entirely in the infrared spectrum, which is why such objects do not visibly glow.

The Spectrum of Thermal Radiation

The emitted thermal radiation spectrum varies with temperature:

• Below $1000 \, \text{K}$: Thermal radiation is 100% infrared. This includes everyday objects like furniture, snow, and the human body, which emit infrared radiation but do not glow visibly.

• Between $1000 \, \text{K}$ and $100,000 \, \text{K}$: Objects in this range emit a combination of infrared, visible light, and ultraviolet radiation. For instance, an incandescent light bulb at $2820 \, \text{K}$ emits approximately $91\%$ infrared radiation and $9\%$ visible light.

• Above $100,000 \, \text{K}$: Thermal radiation includes infrared, visible light, ultraviolet, and x-ray radiation. Hot stars and supernovas exhibit this behavior.

Infrared Radiation

Infrared radiation, on the other hand, specifically encompasses all electromagnetic waves in the wavelength range from $1 \, \text{mm}$ to $650 \, \text{nm}$. It is most commonly produced through thermal processes but can also originate from other sources such as lasers and LEDs.

Key Differences Summarized

To summarize:

• Thermal Radiation: Refers to electromagnetic radiation emitted due to the thermal emission process, regardless of frequency. It can include infrared, visible, and other wavelengths.

• Infrared Radiation: Refers specifically to electromagnetic waves in the infrared range ($0.3 \, \text{THz}$ to $400 \, \text{THz}$), irrespective of how they were generated.

For example:

• The glow from a campfire is visible light thermal radiation, not infrared radiation.
• Signals from a remote control are infrared radiation but do not constitute thermal radiation.
• The radiant heat from a warm rock is both infrared radiation and thermal radiation.

Conclusion

In conclusion, while thermal radiation and infrared radiation are related, they are not synonymous. Thermal radiation encompasses all electromagnetic radiation emitted due to temperature, while infrared radiation is specifically defined by its frequency range. Understanding this distinction helps clarify many concepts in physics regarding heat transfer and electromagnetic radiation.