Thermal Radiation (Edexcel GCSE Physics)

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Katie M

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Thermal Radiation

  • All bodies (objects), no matter what temperature, emit a spectrum of thermal radiation in the form of electromagnetic waves
    • These electromagnetic waves usually lie in the infrared region of the spectrum but could be emitted in the form of visible light or other wavelengths, depending on the temperature

  • The hotter object, the more infrared radiation it radiates in a given time

Leslie-cube, IGCSE & GCSE Physics revision notes

The infrared emitted from a hot object can be detected using a special camera

  • The intensity and wavelength distribution of any emitted waves depends on the temperature of the body
  • This can be represented on a thermal radiation curve
    • As the temperature increases, the peak of the curve moves
    • This moves to a lower wavelength and a higher intensity

Black Body CurveThe peak of a thermal radiation curve moves to the left with increasing temperature

  • From the electromagnetic spectrum, waves with a smaller wavelength have higher energy (e.g. UV rays, X-rays)
  • When an object gets hotter, the amount of thermal radiation it emits increases
  • This increases the energy emitted and therefore the wavelength of the emitted radiation decreases
    • At room temperature objects emit thermal radiation in the infrared region of the spectrum
    • At around 1000 °C an object will emit a significant amount of red light
    • At 6000 °C an object will mainly emit white or blue light (and some ultraviolet)
    • At even higher temperatures objects will emit ultraviolet or even X-rays

Thermal Equilibrium

Higher Tier Only

  • As an object absorbs thermal radiation it will become hotter
  • As it gets hotter it will also emit more thermal radiation
    • The temperature of a body increases when the body absorbs radiation faster than it emits radiation

  • Eventually, an object will reach a point of constant temperature where it is absorbing radiation at the same rate as it is emitting radiation
    • At this point, the object will be in thermal equilibrium

thermal-equilibrium, IGCSE & GCSE Physics revision notes

An object will remain at a constant temperature if it absorbs heat at the same rate as it loses heat

Radiation & Temperature

Higher Tier Only

  • The temperature of a body can be regulated by balancing how much incoming radiation is absorbed and emitted (or reflected)
  • If an object starts to absorb radiation at a higher rate than it radiates it, then the object will heat up
    • Likewise, if it loses radiation at a greater rate than it absorbs it, then the object will cool down

  • This is how an emergency blanket works, to keep a trauma victim warm:
    • Rescue teams use light-coloured, shiny emergency blankets to keep accident survivors warm
    • A light, shiny outer surface emits a lot less radiation than a dark, matt (non-glossy) surface
    • This keeps the patient warm, as less infrared radiation is emitted than if an ordinary blanket had been used

The reflective nature of a emergency blanket helps to keep a trauma patient warm

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.