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[[Thermal radiation]] is [[electromagnetic radiation]] emitted from the surface of an object due to the object's [[temperature]]. If the object is heated sufficiently, it starts to emit light at the red end of the [[spectrum]]: it is ''red hot''. Heating it further causes the color to change, as light of shorter wavelengths (higher frequencies) becomes stronger. A good emitter is also a good absorber. When it is cold, such an object looks black, as it emits practically no visible light, absorbing all light that falls on it. Consequently, its absorbing properties make it an an ideal emitter; this is known as a [[black body]], and the radiation it emits is called black body radiation.
In the late 19th Century, thermal radiation had been fairly well characterised experimentally. <!-- The following text copied from [[Black body]] -->The wavelength at which the radiation is strongest is given by [[Wien's displacement law]], and the overall power emitted per unit area is given by the [[Stefan–Boltzmann law]]. As temperature increases, the glow
<!-- The amount of radiation emitted at all frequencies increases, but the increase is relatively larger at shorter wavelengths, and so the peak in the intensity distribution shifts to shorter wavelengths. -->
Physicists were searching for a theoretical explanation of these experimental results.
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