Thermal Radiation (DP IB Physics): Revision Note

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

• Thermal radiation is defined as:

  Heat transfer by means of electromagnetic radiation normally in the infrared region

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

  • This is because atoms and molecules above absolute zero are in constant motion

  • Electric charges within the atoms in a material vibrate causing electromagnetic radiation to be emitted

  • Therefore, the higher the temperature, the greater the thermal motion of the atoms and the greater the rate of emission of radiation

• Thermal radiation is the only method of thermal energy transfer that does not require matter in order to move or propagate

  • Therefore, thermal radiation is the only way heat can travel through a vacuum

Thermal radiation demonstration on a Leslie cube

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

• 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 (shiny surfaces = less radiation)

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

• Dark, dull objects are better at emitting and absorbing radiation

• Light, shiny objects are worse at emitting and absorbing radiation

Black-Body Radiation

• Black-body radiation is the name given to the thermal radiation emitted by all bodies
  

  • Black-body radiation can be emitted in the form of infrared light, but also visible light or other wavelengths, depending on the temperature

• A perfect black body is defined as:

  An object that absorbs all of the radiation incident on it and does not reflect or transmit any

• Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter too

• As a result, an object which completely absorbs all radiation will be black

  • This is because the colour black is what is seen when all colours from the visible light spectrum are absorbed

• The intensity and wavelength distribution of any emitted waves depends on the temperature of the body

• This can be represented on a black-body radiation curve of intensity against wavelength

  • As the temperature increases, the peak of the curve moves

  • This moves to a lower wavelength and a higher intensity

Black-body radiation curves

Black body spectrum for objects of different temperatures

• From the electromagnetic spectrum, waves with a smaller wavelength have higher energy (e.g. UV rays, X-rays)
  

  • The hotter the object, the greater the amount of infrared radiation it radiates in a given time

• A higher temperature increases the thermal energy emitted and therefore the wavelength of the radiation emitted at the greatest intensity, λ_peak, decreases

  • At room temperature, objects emit thermal radiation in the infrared region of the spectrum (λ_peak is in the infrared region)

  • At around 1000°C, an object will emit a significant amount of red visible light (λ_peak is in the red region of the visible spectrum)

  • At around 6000°C, an object will mainly emit white or blue visible light (λ_peak is in the centre or violet region of the visible spectrum)

  • At even higher temperatures, objects will emit ultraviolet or even X-rays