Thermal radiation is
electromagnetic radiation emitted from the surface of an object which is due to the object's
temperature.
Infrared radiation from a common household
radiator or
electric heater is an example of thermal radiation, as is the
light emitted by a glowing
incandescent light bulb. Thermal radiation is generated when heat from the movement of
charged particles within
atoms is converted to electromagnetic radiation. The emitted wave
frequency of the thermal radiation is a probability distribution depending only on temperature, and for a genuine
black body is given by
Planck’s law of radiation.
Wien's law gives the most likely frequency of the emitted radiation, and the
Stefan–Boltzmann law gives the heat intensity.
There are three main properties that characterize thermal radiation
Thermal radiation is an important concept in thermodynamics as it is partially responsible for heat exchange between objects, as warmer bodies radiate more heat than colder ones. (Other factors are convection and conduction.) The interplay of energy exchange is characterized by the following equation
Here, 
represents spectral absorption factor, 
spectral reflection factor and 
spectral transmission factor. All these elements depend also on the wavelength 
. The spectral absorption factor is equal to the emissivity 
; this relation is known as Kirchhoff's law of thermal radiation. An object is called a black body if, for all frequencies, the following formula applies
