Thermal radiation is the transfer of energy by infrared electromagnetic waves. Unlike conduction and convection, radiation does not require a medium and can travel through a vacuum.
Key properties
- All objects emit and absorb thermal radiation.
- Hotter objects emit more radiation per second.
- Radiation travels at the speed of light.
- It is the only method of thermal energy transfer that works through a vacuum (this is how the Sun’s energy reaches the Earth).
Surface effects
The rate of emission and absorption depends on the surface:
| Surface | Emission | Absorption |
|---|---|---|
| Dark, matt (rough) | Good emitter | Good absorber |
| Light, shiny (smooth) | Poor emitter (good reflector) | Poor absorber (good reflector) |
Applications
| Application | Surface used | Reason |
|---|---|---|
| Solar water heater | Matt black panel | Absorbs maximum radiation from the Sun |
| Survival blanket | Shiny silver | Reflects body radiation back; reduces emission |
| Radiator (central heating) | Painted white or cream | Despite the name, radiators mainly heat by convection; the paint colour has a small effect |
| Kettle | Shiny metallic | Reduces radiation emission, keeping water hot longer |
Worked example
Two identical cans, one painted matt black and one painted shiny silver, are filled with equal amounts of hot water. Explain which can cools faster.
The matt black can cools faster because a dark, matt surface is a good emitter of infrared radiation. It radiates thermal energy at a higher rate than the shiny silver can, which is a poor emitter.
Common errors and how to correct them
Confusing radiation with convection or conduction. Radiation is energy transfer by electromagnetic waves. It does not need particles or a medium.
Saying “black absorbs heat and white reflects heat.” Be specific: dark, matt surfaces absorb and emit infrared radiation well. Light, shiny surfaces reflect infrared radiation and are poor emitters. The key properties are colour and texture (matt vs shiny), not colour alone.
How to apply this in an exam
When explaining radiation, always specify infrared radiation (not just “radiation,” which could be confused with nuclear radiation). State both colour and texture (dark, matt vs light, shiny) when describing surface effects.
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