Know that thermal radiation is infrared radiation and that all objects emit this radiation
2.3.3 Radiation – Thermal Radiation & Infrared
What is Thermal Radiation?
Every object that has a temperature above absolute zero emits energy in the form of electromagnetic waves. This energy is called thermal radiation and it is always in the infrared part of the spectrum. 🔥🌡️
Think of a campfire: the heat you feel from the flames is mainly due to infrared radiation, even though you cannot see it.
Infrared Radiation
Infrared waves have wavelengths longer than visible light but shorter than microwaves:
| Region | Wavelength Range |
|---|---|
| Visible | $400-700\,\text{nm}$ |
| Infrared | $700\,\text{nm} - 1\,\text{mm}$ |
| Microwave | $1\,\text{mm} - 1\,\text{m}$ |
Notice how the infrared range starts just beyond the visible spectrum.
All Objects Emit Radiation
Even a cold object emits infrared radiation. The amount of radiation depends on its temperature.
Analogy: A phone screen glows faintly in the dark because it emits a tiny amount of infrared light.
The total power emitted by a surface is given by the Stefan‑Boltzmann law:
$P = \sigma A T^4$
- $\sigma$ is the Stefan‑Boltzmann constant ($5.67\times10^{-8}\,\text{W m}^{-2}\text{K}^{-4}$)
- $A$ is the surface area
- $T$ is the absolute temperature (in Kelvin)
Example: A 20 °C room (~293 K) emits more infrared power than a 5 °C room (~278 K).
Exam Tips 📚
- ?? Remember: Thermal radiation is always infrared.
- ?? All objects emit IR, regardless of temperature.
- ?? Use the Stefan‑Boltzmann law to calculate power.
- ?? Know the wavelength ranges of visible, infrared, and microwave.
- ?? Intensity increases with the fourth power of temperature ($T^4$).
Good luck on the exam! 🎓
Revision
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