Here, a brief description of three modes of heat transfer is presented. Conduction occurs in all media-solids, liquids, and gases when a temperature gradient exists. In opaque solids, it is the only mechanism by which the heat can flow. In convection, transport of heat takes place as a volume of liquid or gas moves from a region of one temperature to that of another temperature. The thermal radiation is the process of heat propagation by means of electromagnetic waves produced by virtue of the temperature of the body. In contrast to the conduction and convection heat transfers, radiation can take place through a perfect vacuum.