Radiation

Radiation is the transfer of heat energy from one place to another by the EMISSION of electromagnetic (EM) waves -- travelling in straight lines at the speed of light, $3\times10^8\text{ m/s}$ -- WITHOUT heating whatever lies in between. Unlike conduction and convection, radiation needs absolutely no material medium at all; it is the only one of the three mechanisms that can carry heat across a true vacuum, which is exactly how the Sun's heat crosses empty space to reach the Earth's atmosphere.

Every object above absolute zero possesses thermal energy by virtue of its temperature, and the rapidly moving/vibrating molecules of a hot body constantly emit EM waves -- thermal radiation -- carrying energy away with them. When this radiation falls on a cooler body, it is absorbed, increasing that body's own molecular motion and hence its temperature. Seen this way, radiative heat transfer is a genuinely two-step process: thermal (molecular) energy is first converted into EM-wave energy by the emitting (hot) body, then reconverted back into thermal …