Planck's law

<p>In <a href="/facts/Physics/a7aH2y08">physics</a>, Planck's law (also Planck radiation law: 1305 ) describes the <a href="/facts/Spectrum_(physical_sciences)/lY3MxemE">spectral density</a> of <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a> emitted by a <a href="/facts/Black_body/f0fHghDy">black body</a> in <a href="/facts/Thermal_equilibrium/wHqArIwx">thermal equilibrium</a> at a given <a href="/facts/Temperature/QyGe4gmj">temperature</a> T, when there is no net flow of <a href="/facts/Matter/osyc04UN">matter</a> or <a href="/facts/Energy/wd8PrQM7">energy</a> between the body and its environment.
</p><p>At the end of the 19th century, physicists were unable to explain why the observed spectrum of <a href="/facts/Black-body_radiation/K4YKPKXL">black-body radiation</a>, which by then had been accurately measured, diverged significantly at higher <a href="/facts/Frequencies/QUSbPaW8">frequencies</a> from that predicted by existing theories. In 1900, German physicist <a href="/facts/Max_Planck/KKk0698y">Max Planck</a> heuristically derived a formula for the observed spectrum by assuming that a hypothetical electrically charged <a href="/facts/Oscillation/6vSrm84a">oscillator</a> in a cavity that contained black-body radiation could only change its <a href="/facts/Energy/wd8PrQM7">energy</a> in a minimal increment, E, that was proportional to the frequency of its associated <a href="/facts/Electromagnetic_wave/Ynx7ujCW">electromagnetic wave</a>. While Planck originally regarded the hypothesis of dividing energy into increments as a mathematical artifice, introduced merely to get the correct answer, other physicists including <a href="/facts/Albert_Einstein/CNTet8qu">Albert Einstein</a> built on his work, and Planck's insight is now recognized to be of fundamental importance to <a href="/facts/Quantum_mechanics/BRc3Mzgr">quantum theory</a>.
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Planck's law open-in-new

Planck's law