Radiation (Cambridge (CIE) O Level Physics): Revision Note

Thermal Radiation

• All objects give off thermal radiation

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

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

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

  • It is the way in which thermal energy 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

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

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

  • The texture of the surface (dull surfaces emit and absorb more radiation than shiny ones)

  • The surface area of the object (greater surface area = more area for radiation to be emitted from)

Absorbing and Emitting Powers of Different Surfaces

• 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 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

Leslie Cube Emission

An image of a hot object taken in both Infrared and visible light. The black surface emits more thermal radiation (infrared) than the shiny surface