Continuous emission (Black body radiation)
All normal matter emits electromagnetic radiation when it has a temperature above
absolute zero. The radiation represents a conversion of a body's internal energy into
electromagnetic energy, and is therefore called thermal radiation.
Conversely, normal matter absorbs electromagnetic radiation to some degree. An object
that absorbs all radiation falling on it, at all wavelengths, is called a black body. When a
black body is at a constant temperature, its emission has a characteristic frequency
distribution that depends on the temperature. Its emission is called black-body radiation.
When a black body is heated and its temperature raises, the intensity of the radiation it
emits increases, and the maximum of the wavelength of emission is shifted towards
shorter wavelengths. The following Figure illustrates the dependence of the intensity of
the black body radiation on temperature and the distribution of the wavelength in the
light emitted at different temperatures. The maximum wavelength of the radiation, which
is in the IR range of the electromagnetic spectrum shifts to the visible range and
become seen.
.
Figure 1: Distribution of wavelengths in
black body irradiation as function of
temperature
Figure 2: An Active volcan in 'action'. Different
colors indicating different temperatures of the lava