Emission and absorption of infrared radiation

All bodies (objects) emit and absorb infrared radiation. They do this whatever their temperature. The hotter the body:

  • the more infrared radiation it gives out in a given time
  • the greater the proportion of emitted radiation is visible light

Black bodies

There are no known objects that are perfect at absorbing or emitting all the radiation, of every possible frequency, that may be directed at it. Some objects do, however, come close to this and these are referred to as "black bodies".

Jonny Nelson introduces an animated explanation of black body radiation

A perfect black body is a theoretical object. It would have these properties:

  • it would absorb all the radiation that falls on it
  • it would not reflect or transmit any radiation

An object that is good at absorbing radiation is also a good emitter, so a perfect black body would be the best possible emitter of radiation.

A black sphere - with all the colours in the spectrum hitting the object. These arrows represent radiation of any wavelength hitting the object.Features of a perfect black body

Stars are considered to be black bodies because they are very good emitters of most wavelengths in the electromagnetic spectrum. This suggests that stars also absorb most wavelengths. Whilst there are a few wavelengths that stars do not absorb or emit, this figure is very low, so they can be treated as black bodies. Planets and black holes are also treated as nearly perfect black bodies.

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.