Radiation (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

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

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

Colour/texture	Absorbing	Emitting
Black	Good absorber	Good emitter
Dull/dark	Reasonable absorber	Reasonable emitter
White	Poor absorber	Poor emitter
Shiny	Very poor absorber (it reflects)	Very poor emitter

• 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)