Everything warmer than absolute zero is quietly shining, including you
Every object above absolute zero gives off electromagnetic radiation simply because its particles jiggle. At room temperature that glow is infrared and invisible, which is why thermal cameras can spot a person in the dark. Heat something past roughly 525 degrees Celsius and the glow creeps into visible light.
Heat moves in three ways: conduction, convection and radiation. The last needs no medium at all. Atoms and molecules are built from charged particles, and as they collide and vibrate those charges accelerate, producing electric and magnetic fields that carry energy away as photons. That is how the Sun warms Earth across empty space; atmospheric scattering of some of that sunlight is also why the sky looks blue.
Physicists model the ideal case with a black body, a perfect absorber that is also a perfect emitter. Planck's law gives its spectrum, and Wien's displacement law says the peak shifts with temperature. The Sun's surface, at around 6000 K, peaks in visible light; Earth's surface, near 300 K, radiates in the infrared. The atmosphere lets much sunlight in but absorbs most of that outgoing infrared, with only about 10 percent escaping directly to space. That selectivity drives the greenhouse effect. A useful rule follows from equilibrium: good absorbers are good emitters, and poor absorbers are poor emitters.
People exploited radiant heat long before explaining it. Burning glasses go back to about 700 BC, and Aristophanes mentioned one in The Clouds in 423 BC. The tale of Archimedes setting Roman ships alight with mirrors has no confirmed contemporary source. In 1589 Giambattista della Porta used a concave metal mirror to feel a distant candle's heat on his face, and even the Holy Roman Emperor Rudolf II repeated the demonstration.
Benjamin Franklin laid coloured cloths on snow in sunshine and found the black squares sank deepest. In 1790 Marc-Auguste Pictet seemed to focus cold with mirrors, and his colleague Pierre Prevost answered in 1791 that all bodies constantly radiate and absorb heat, cooler ones simply taking in more than they give out. William Herschel, passing sunlight through a prism in 1800, found a thermometer warming beyond the red end of the spectrum: the discovery of infrared.
Source: Thermal radiation
