Radiation occurs in the form of electromagnetic waves. It is the emission of energy, or heat in this case, through space or a material medium. Radio waves, microwaves, and visible light are all forms of electromagnetic radiation. These applications are in our everyday lives. When one listens to music or heats up food, electromagnetic waves are emitted. Other than electromagnetic radiation, there is also thermal radiation in which refers to the emission of heat. When one sits next to a campfire, he/she will feel warmer due to the transfer of heat via thermal radiation. The sunlight necessary for life to exist is considered the thermal radiation from the Sun. Earth also emits thermal radiation, however, less in intensity compared to the Sun.
The equation above describes the emissive power of a black body. ? is the Stefan-Boltzmann constant, 5.67E-8 watt/(m2k-4) and T is the temperature in °K. ? is the emissivity which is emissive power of a body over the emissive power of a black body. Black body is the ideal body that absorbs all incident radiation, regardless of frequency or angle of incidence. As a result, non-black bodies do not absorb all of the radiant energy that hits them. This is why bodies with emissivity closer to unity, or 1, are good absorbers and poor reflectors of radiation. In addition, highly polished and unoxidized metal surfaces are great reflectors of radiation due to their emissivity being less than 0.1. Rough and oxidized metal surfaces, on the other hand, are great absorbers of thermal radiation. Nonmetallic surfaces have a emissivity above 0.8. The heat exchange between two surfaces is given in the equation below. F is a variable that takes into account the geometric relationship of the two surfaces and the emissivity and absorptivity of surfaces.