Thermal radiation involves energy transferred via electromagnetic waves. Often this is infrared radiation, but it can also be visible light or radiation of higher energy.
Thermal radiation is relatively safe, and is not the dangerous nuclear radiation associated with nuclear bombs, etc.
All objects continually absorb thermal energy and radiate it away again. When everything is at the same temperature, the amount of energy received is equal to the amount given off and no changes in temperature occur. If an object emits more than it absorbs, though, it cools down.
For an object with a temperature T (in Kelvin) and a surface area A, the net rate of radiated energy depends strongly on temperature:
Pnet = Prad - Pabs = seA(T4 - Tenv4)
where Tenv is the temperature of the surrounding environment, and
the Stefan-Boltzmann constant s
= 5.67 x 10-8 W/m2
e is known as the emissivity. It is a measure of how efficiently an object absorbs and emits radiated energy. Highly reflective objects have emissivities close to zero. Black objects have emissivities close to 1. An object with e = 1 is called a perfect blackbody,
The best absorbers are also the best emitters. Black objects heat up faster than shiny ones, but they cool down faster too.