To take into account the fact that real objects are gray bodies, the Stefan-Boltzmann law must include emissivity.
They radiate less heat than a black body and therefore are called gray bodies. Real objects do not radiate as much heat as a perfect black body. The emissivity, ε, of the surface of a material is its effectiveness in emitting energy as thermal radiation and varies between 0.0 and 1.0.īy definition, a blackbody in thermal equilibrium has an emissivity of ε = 1.0. For example, solar radiation reaches the surface of the earth after passing through cold layers of atmosphere at high altitudes. In contrast to heat transfer by conduction or convection, which take place in the direction of decreasing temperature, thermal radiation heat transfer can occur between two bodies separated by a medium colder than both bodies. In fact, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum. Thermal radiation does not require any medium for energy transfer.
It is generated by the thermal motion of charged particles in matter and therefore any material that has a temperature above absolute zero gives off some radiant energy. The term thermal radiation is frequently used to distinguish this form of electromagnetic radiation from other forms, such as radio waves, x-rays, or gamma rays. Thermal radiation is electromagnetic radiation in the infra-red region of the electromagnetic spectrum although some of it is in the visible region.
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