Glossary / History of physics
Blackbody radiation
Light emitted by an ideal body that absorbs all light falling on it. Its spectrum depends only on temperature, not on what the body is made of.
In 1860 Gustav Kirchhoff showed that the spectrum emitted by a perfectly absorbing body is universal, set by temperature alone[1]. What formula describes it became a central problem of late-nineteenth-century physics, and precise measurements advanced partly for practical reasons such as controlling furnace temperatures.
Formulas derived from existing physics disagreed badly with measurement at short wavelengths. In 1900 Max Planck obtained a formula that fitted by assuming energy comes only in discrete amounts[2]. Quantum theory began with this problem.
Related terms
- Planck constantThe fundamental constant, written h, that sets the step size of energy for light and matter. Frequency times h is the smallest unit of energy for that light.
- Light quantumThe particle-like packet of energy that light behaves as. Now called the photon.
- Old quantum theoryThe theories of roughly 1900 to 1925, from Planck onwards, that explained phenomena by adding quantum conditions on top of classical physics.