Radiation (Cambridge (CIE) IGCSE Physics): 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 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

How do surface colour and texture affect infrared radiation?

• The amount of thermal radiation emitted by an object (opens in a new tab) 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, emitting and reflecting 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

Comparing emission from different surfaces

Thermal equilibrium

Extended Tier Only

• As an object absorbs thermal radiation it will become hotter

  • As it gets hotter it will also emit more thermal radiation

• Eventually, the object reaches a point where it transfers energy away at the same rate as it receives energy

  • At this point, the object will be at a constant temperature

  • An object at a constant temperature is in thermal equilibrium

Heating & cooling by energy transfer

Extended Tier Only

• If the rate at which an object receives energy is greater than the rate at which it transfers energy away:

  • the object's temperature will increase

• If the rate at which an object transfers energy away is greater than the rate at which it receives energy:

  • the object's temperature will decrease

• The process will always move towards thermal equilibrium

  • As temperature increases, the object emits more radiation until the rates are equal

  • As temperature decreases, the object transfers less energy until the rates are equal