It is the process in which transfer of energy takes place in the form of wave from one place to another through a medium or space.

`text(Some important points about the thermal radiation :)`

i. Everybody radiates electromagnetic radiation of all possible wavelengths at all temp > 0K

ii. Medium not required for the propagation of these.

iii. Their intensity is inversely proportional to the square of distance of point of observation from the source.

`I=1/r^2`

`text(PREVOST THEORY OF HEAT EXCHANGE)`

i. Everybody emits heat radiations at all finite temperatures (except 0 K) as well as it absorbs radiation from the surroundings. It is a continuous phenomenon.

ii. If the amount of radiation absorbed by a body is greater than, amount of radiation emitted then temperature of body will increase, or if the amount of radiation absorbed is less than amount of radiation emitted, then temperature of body decreases. This is a continuous phenomenon, till body comes to thermal equilibrium with surrounding.

`text(PERFECTLY BLACK BODY)`
A perfectly black body is that, which absorbs all the radiations of all wave lengths incident on it As a perfectly black body it neither reflects nor transmits any radiation.

When perfectly black body is heated to suitable high temperature, it emits all type of radiations of all possible wavelengths.