Black Body Radiation
This section explains black body radiation covering, emission and absorption of infrared radiation, the surface properties and infrared radiation and black body radiation and temperature.
Black Body Radiation
Black body radiation refers to the thermal radiation emitted by an object due to its temperature. A perfect black body is an object that:
- Absorbs all radiation that falls on it (no reflection or transmission).
- Emits radiation perfectly, depending only on its temperature.
Although perfect black bodies do not exist in reality, some objects (such as the Sun) behave similarly.
Emission and Absorption of Infrared Radiation
Infrared Radiation (IR) and Temperature
All objects emit and absorb infrared radiation (a type of electromagnetic wave) continuously. The amount of IR emitted depends on the temperature and surface properties of the object:
- Hotter objects emit more infrared radiation.
- Cooler objects absorb more infrared radiation than they emit (if surrounded by hotter objects).
- When an object reaches thermal equilibrium, it emits and absorbs radiation at the same rate.
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Surface Properties and Infrared Radiation
The ability to emit and absorb infrared radiation depends on an object’s surface:
| Surface Type | Absorption of IR | Emission of IR |
|---|---|---|
| Black, matt (dull) surfaces | Best absorbers | Best emitters |
| White or shiny surfaces | Poor absorbers (reflect most IR) | Poor emitters |
This explains why:
- Black objects heat up faster in sunlight.
- Shiny materials (like foil) are used for thermal insulation as they reflect infrared radiation.
Black Body Radiation and Temperature
The radiation emitted by a hot object includes a range of wavelengths. As temperature increases:
- The intensity of emitted radiation increases.
- The peak wavelength of emitted radiation shifts towards the shorter wavelengths (from infrared towards visible light).
- Cooler objects emit mostly infrared radiation.
- Very hot objects (like stars) emit visible light (appearing red, then white as they get hotter).
This is why metal glows red when heated and why stars appear different colours depending on their temperature.
Summary
- All objects emit and absorb infrared radiation.
- The amount of IR emitted depends on temperature and surface properties.
- Black, matt surfaces are the best absorbers and emitters of IR.
- Shiny surfaces reflect infrared and are poor emitters.
- As temperature increases, radiation intensity increases and peak wavelength decreases.
This concept is crucial in understanding how objects cool, heat up, and radiate energy in their surroundings.
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