Emission and absorption of radiation

All bodies (objects) emit electromagnetic radiation, no matter what their temperature is. The type of radiation emitted depends on the temperature of the body.

Hot bodies emit a continuous range of electromagnetic radiation at different energy values - this means that the radiation that is emitted is spread out over a range of different frequencies and wavelengths.

As a body gets hotter, it emits more high frequency electromagnetic radiation than low frequency. The higher the frequency, the shorter the wavelength. This means that the intensity of the radiation emitted is greater for a hotter body.

As the surface temperature of a hot body such as a star increases:

  • the intensity of the radiation it emits increases
  • the wavelength that corresponds to the peak intensity gets shorter

Poor absorbers and emitters

White and shiny silvery surfaces are the worst absorbers, as they reflect all visible light wavelengths. Poor absorbers are also poor emitters, and do not emit radiation as quickly as darker colours. Radiators in homes are usually painted white so that the infrared radiation is emitted gradually.

Jonny Nelson introduces an animated explanation of black body radiation