Revision Notes

2.3.3 Radiation

Radiation: Basics

  • All hot objects give off thermal radiation: the hotter they are, the more they emit
  • Thermal radiation is part of the electromagnetic spectrum – infrared
  • 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
  • The colour of an object affects how good it is at emitting and absorbing thermal radiation:

Radiation table, IGCSE & GCSE Physics revision notes

  • Black objects are very good at absorbing thermal radiation (think about black leather seats in strong sunshine) but also very good at emitting it (when it goes dark those seats cool down quickly)
  • Shiny objects reflect thermal radiation and so absorb very little. They also emit very little, though, and so take longer to cool down


Demonstrating radiation, IGCSE & GCSE Physics revision notes

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


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Factors Affecting Emission of Radiation

  • The amount of thermal radiation emitted by an object depends on a number of factors:
    • The temperature of the object (hotter = more radiation)
    • The colour of the object (black = more radiation)
    • The surface area of the object (greater surface area = more area for radiation to be emitted from)
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Demonstrating Radiation

  • To demonstrate the absorption of thermal radiation:
    • Take two conical flasks – one painted with silver paint, the other with black paint – and place thermometers and bungs in them
    • Measure and record their initial temperatures
    • Place the two flasks an equal distance from an incandescent light bulb (a good source of radiation) and switch the bulb on
    • After a few minutes (between 2 and 5) switch the bulb off and record the new temperatures of the flasks
      (The black flask’s temperature should have increased by more

shiny-black-beakers, IGCSE & GCSE Physics revision notes

A black and a shiny beaker can be used to demonstrate the effect of colour upon the emission and absorption of thermal radiation


  • To demonstrate the emission of thermal radiation:
    • Fill the shiny beakers with boiling water
    • Once each beaker reaches a set temperature (e.g. 90 °c) start a stopwatch and allow it to cool for a set amount of time (e.g. 10 minutes)
    • After this time, take a new temperature measurement and record the change in temperature
      (The black beaker should have cooled by slightly more than the shiny beaker, because it emitted more thermal radiation)


Exam Tip

If a question on heat mentions the colour of something (black, white or shiny) then the answer will probably be about thermal radiation.

Shiny things do not reflect heat (they reflect thermal radiation).

Black things do not absorb heat (they absorb thermal radiation).

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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