A blackbody is an idealized object that absorbs all radiation and emits a characteristic spectrum depending only on its temperature.A blackbody is an idealized physical object that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence, and re-emits it in a spectrum that depends solely on its temperature. This spectrum, known as the Planck spectrum, is a continuous curve with a characteristic peak that shifts to shorter wavelengths as temperature increases. The concept is fundamental in physics because it provides a universal benchmark for understanding thermal radiation, from the glow of a hot metal to the cosmic microwave background.
The concept was introduced by Gustav Kirchhoff in 1860, and the theoretical explanation of the blackbody spectrum by Max Planck in 1900 marked the birth of quantum mechanics.
In a perfect blackbody, no radiation is reflected or transmitted; all energy is absorbed and then re-emitted in thermal equilibrium. The emitted spectrum is described by Planck's law, which quantifies the intensity of radiation at each wavelength as a function of temperature. Real objects approximate blackbodies to varying degrees, with their emissivity (a factor between 0 and 1) indicating how closely they match the ideal. Stars, including our Sun, are often approximated as blackbodies to estimate their surface temperatures from their spectra.
Blackbody models are used in astrophysics to determine stellar temperatures, in engineering for designing thermal radiators and sensors, and in metrology for calibrating infrared thermometers and cameras.
The cosmic microwave background radiation, left over from the Big Bang, is the most perfect blackbody spectrum ever measured, with a temperature of 2.725 Kelvin.
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