Radiation (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Thermal radiation

What is thermal radiation?

• All objects give off thermal radiation

  • The hotter an object is, the more thermal radiation it emits

  • Thermal radiation is mainly due to infrared radiation (part of the electromagnetic spectrum)

• Thermal radiation is the only way in which heat can travel through a vacuum

  • It is the way in which heat reaches us from the Sun through the vacuum of space

  • Thermal radiation does not need a medium to travel, unlike convection and conduction

Effects of different surfaces

What two factors affect the rate of thermal energy transfer?

• The amount of thermal radiation emitted by an object depends on a number of factors:

  • The surface colour of the object (black = more radiation emitted and absorbed)

  • The texture of the surface (shiny surfaces = less radiation emitted and absorbed)

Table of the effect of coloured surfaces on absorbing and emitting ability

• Black objects are very good at absorbing thermal radiation, for example black clothes make you feel hotter in sunny weather

  • Black objects are also very good at emitting thermal radiation, which is the reason that some chargers for laptops, and radiators in cars are coloured black - it helps them to cool down

• Shiny objects reflect thermal radiation and so absorb very little

  • They also emit very little, though, and so take longer to cool down

The infrared emitted from a hot object can be detected using a special camera. The dull black side of the cube (left) is seen to glow brighter with infrared radiation that the shiny light side of the cube (right)