In a gas, photons will be emitted in the process of energy transitions of individual
electrons. The wavelengths of such photons for a specific gas can only have a limited
number of discrete values, each value being related to a certain type of energy
In a solid, comparable transitions of individual electrons can take place but for the
major part the exchange of energy is done by vibrational transitions of atoms within
the grid they are bound in. Increasing the applied energy will cause transitions from
vibrations with a lower level of energy to vibrations with a higher level of energy. The
material will release its energy with opposing transitions from higher to lower levels of
energy and a simultaneous release of photons. The type and intensity of vibrational
transitions in a solid will change with rising temperatures, proportional to the amount
of applied energy.
With radiators operating at low levels of energy per square area, all of the radiation
will be of the long- and mid range infrared type and no visible light will be emitted.
These kind of radiators are called dull radiators. Radiators working at higher levels of
energy will have a higher operating temperature and, beside infrared radiation, an
increasing amount of light will be emitted, starting with red (red-glowing radiators) to
eventually white (white-glowing radiators).