This occurs by the process of radiation.
Thermal radiation is a form of electromagnetic radiation like light.
It travels in straight lines, can be reflected and its intensity obeys the inverse square law. It can be refracted by a silica prism, demonstrating that it has a longer wavelength that visible light. The wavelength of the brightest part of the emission spectrum decreases with increasing temperature. Detection of Radiation Thermal radiation can be detected by a number of instruments. These all rely on detecting the difference in temperature between a blackened detector, which efficiently absorbs the radiation, and a similar detector which is either silvered, to reflect the radiation, or shielded, so as not to be exposed to it.