Basic concepts of quantum mechanics: Difference between revisions

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==Origins in black body radiation==

[[Image:Hot metalwork.jpg|300px|thumb|right|Hot metalwork from a blacksmith. The yellow-orange glow is the visible part of the thermal radiation emitted due to the high temperature. Everything else in the picture is glowing with thermal radiation as well, but less brightly and at longer wavelengths that the human eye cannot see. A far-infrared camera records this radiation.]]

[[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 color changes from red to yellow to white to blue. Even as the peak wavelength moves into the ultra-violet,enough radiation continues to be emitted in the blue wavelengths that the body continues to appear blue. It never becomes invisible—indeed, the radiation of visible light increases [[monotonic function|monotonically]] with temperature.<ref name="Landau">{{cite book |last=Landau |first=L. D.|coauthors=E. M. Lifshitz |title=Statistical Physics |edition=3rd Edition Part 1 |year=1996 |publisher=Butterworth-Heinemann |___location=Oxford}}</ref>