Emission and absorption of radiation

All bodies (objects) emit and absorb types of electromagnetic radiation. They do this regardless of their temperature.

The intensity of radiation increases as the body gets hotter and gives out more radiation in a given time. The type of radiation emitted also changes with temperature.

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. Here is the intensity-wavelength graph for the Sun.

A graph showing the intensity of radiation that each wavelength gives off. Visible light gives the most radiation.

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

  • the intensity of radiation it emits increases
  • the wavelength that corresponds to the peak intensity gets shorter
Graph showing the intensity of radiation at each frequency against wavelength. The graph depicts measurements for the coolest star, the sun and the hottest star.

The graph for a hot star, such as a blue supergiant, peaks over a shorter wavelength than a cooler star such as a red giant. The intensity of radiation for each frequency is higher for a very hot star than for a cooler one.

Black bodies

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.

There are no known objects that are perfect at absorbing or emitting all 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.

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.