Edexcel iGCSE Physics -4.8 Radiation, Absorption, and Emission- Study Notes- New Syllabus

Emission and Absorption of Thermal Radiation

Thermal radiation is the transfer of energy by electromagnetic waves, mainly infrared radiation. All objects emit and absorb thermal radiation, and the amount depends on the surface properties and the temperature of the object.

Effect of Surface on Radiation

The ability of a surface to emit or absorb radiation depends strongly on its colour and texture.

• Black, dull surfaces:
  • Very good emitters of radiation
  • Very good absorbers of radiation
• White, shiny surfaces:
  • Poor emitters of radiation
  • Poor absorbers of radiation
  • Good reflectors of radiation

This is why radiators are often painted matt black and survival blankets are shiny.

Effect of Temperature on Radiation

• All objects emit radiation all the time.
• The higher the temperature, the greater the rate of radiation emitted.
• Hot objects emit more radiation than cool objects.

An object emits radiation at a greater rate when it is hotter than its surroundings.

Emission, Absorption and Thermal Equilibrium

• If emission > absorption → object cools down
• If absorption > emission → object heats up
• If emission = absorption → temperature stays constant

This state is called thermal equilibrium.

Everyday Applications

• Black clothes keep you warmer in sunlight.
• Shiny kettles reduce heat loss.
• Space blankets reflect body heat.
• Solar panels are dark to absorb radiation efficiently.

Key Idea

• Surface colour and texture affect radiation.
• Temperature controls the rate of emission.
• Black, dull and hot objects are the best emitters.

Important Points to Remember

• Radiation does not need a medium.
• Emission and absorption are linked.
• Good absorbers are also good emitters.