Radiation
Thermal radiation is the transfer of heat energy by electromagnetic waves — mainly infra-red radiation. Unlike conduction or convection, it does not require a medium for transfer.
Key Features
- Occurs through infra-red radiation.
- Can take place even in a vacuum.
- All objects emit and absorb infra-red radiation.
- Hotter objects emit more radiation per second.
Special Cases
Rate of Emission vs Absorption
- If emission rate > absorption rate → object cools down.
- If emission rate = absorption rate → temperature remains constant.
- If emission rate < absorption rate → object warms up.
Infrared Radiation
- Part of the electromagnetic spectrum.
- Every object emits infra-red radiation — hotter objects emit more.
- Very hot objects also emit visible light along with IR radiation.
- Infra-red radiation travels at the speed of light and does not need particles.
Experiment — Detecting Infrared Radiation
When a glass prism splits white light into a spectrum, a thermometer placed just beyond the red end shows a temperature rise. This invisible region is infra-red radiation, having a lower frequency than red light.
Effect of Surface Colour and Texture
| Surface Type | Absorption | Emission | Reflection |
|---|---|---|---|
| Dark, Dull, Matt Surface | Good absorber | Good emitter | Poor reflector |
| Light, Shiny Surface | Poor absorber | Poor emitter | Good reflector |
Demonstration — Absorbers and Emitters
Two beakers are painted differently — one shiny silver, the other dull black. Both are filled with equal hot water. The black beaker cools faster, proving it is a better emitter and absorber of infra-red radiation.
Controlling Heat Absorption and Emission
- In hot countries, houses are painted white to reflect more radiation.
- White blinds or shutters reduce radiation absorption.
- Thick walls prevent excessive conduction of heat inside buildings.
Applications — Thermos Flask
- Silvered surfaces reflect heat radiation.
- Vacuum layer prevents conduction and convection.
- Plastic stopper acts as an insulator to reduce conduction.
Factors Affecting Emission Rate
- Surface temperature — higher temperature → greater emission.
- Surface area — larger area → faster radiation.
- Colour and texture — dark and dull → better emitters.
Earth’s Temperature and Radiation Balance
Daytime
- Earth’s surface absorbs large amounts of solar radiation.
- Temperature rises as absorbed radiation exceeds emission.
- Clear skies increase warming as less energy is reflected.
Night-time
- More radiation is emitted than absorbed → temperature falls.
- Clear nights cause rapid cooling (no clouds to reflect radiation back).
- Cloudy nights stay warmer as clouds reflect infrared radiation back to Earth.
Overall, this balance maintains a fairly constant temperature on Earth, allowing life to survive.
Exam Tip
Radiation differs from conduction and convection because no particles or medium are required. The efficiency of absorption and emission depends strongly on surface colour, texture, and area.
Explore more at Smart Exam Resources.