Physics

Why everything warm is giving off light

Every object above absolute zero gives off light: heat makes its charged particles vibrate, and vibrating charges radiate. Temperature only decides the colour. A body at 37 degrees Celsius peaks near 9 micrometres, well past the red edge of human vision, while the Sun's surface peaks in visible light. That is why a warm room looks dark.

Sources & further reading

OpenStax, Rice University6.1 Blackbody Radiation (University Physics Volume 3)A free university physics chapter on why every warm object gives off light, how much it gives off, and what sets the colour.Institute of Physics (IOPSpark)William Herschel and the discovery of infra-red radiationThe UK Institute of Physics account of the 1800 experiment that found invisible light past the red end of the rainbow.Caltech / IPAC, Cool CosmosThe Herschel ExperimentCaltech's infrared-astronomy education page, which walks through repeating Herschel's experiment yourself.NASA (Spinoff 2007)Infrared Imaging Sharpens View in Critical SituationsNASA's technology-transfer report on thermal cameras that let firefighters pick out body heat through smoke and darkness.NASASunshield (James Webb Space Telescope)NASA's page explaining why an infrared telescope has to be kept colder than the things it is looking at.Nature (Nature Publishing Group)Molecular basis of infrared detection by snakesThe study that found the molecular heat sensor letting snakes read the body warmth of prey in the dark.ENERGY STAR (US EPA and US Department of Energy)Learn About LED LightingThe US government's efficiency programme, stating how much of an old bulb's power leaves as heat rather than visible light.US Department of EnergyDOE Finalizes Efficiency Standards for Lightbulbs to Save Americans Billions on Household Energy BillsThe US Energy Department announcement behind the rule that retired the traditional light bulb.