{{short description|Temperature above which all solids glow due to black-body radiation}}
In physics, the '''Draper point''' is the approximate temperature above which almost all solid materials visibly glow as a result of black-body radiation. It was established at {{convert|977|°F|°C K}} by John William Draper in 1847.<ref> {{cite journal | title = On the Production of Light by Heat | first = John William |last = Draper | journal = The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science | volume = 30 | year = 1847 | issue = 202 | page = 345–359 | series=Series 3 | doi = 10.1080/14786444708647190 | url = https://books.google.com/books?id=VEgEAAAAYAAJ&pg=PA345 }}</ref><ref> {{cite journal | journal = The Academy | title = Science: Draper's Memoirs | volume = 14 | issue = 338 | first = John William | last = Draper | publisher = Robert Scott Walker |location=London | date = Oct 26, 1878 | page = 408 | url = https://books.google.com/books?id=hZINAAAAQAAJ&q=draper+point+red+1847&pg=PA408 }}</ref><ref> {{cite book | title = Radiation heat transfer: a statistical approach | first = J. Robert |last = Mahan | edition = 3rd | publisher = Wiley-IEEE | year = 2002 | isbn = 978-0-471-21270-6 | page = 58 | url = https://books.google.com/books?id=y9zUEzA7iN0C&q=draper-point+red&pg=PA58 }}</ref>
Bodies at temperatures just below the Draper point radiate primarily in the infrared range and emit negligible visible light. The value of the Draper point can be calculated using Wien's displacement law: the peak frequency <math>\nu_\text{peak}</math> (in hertz) emitted by a blackbody relates to temperature as follows:<ref>{{cite book |title=Statistical Physics |last=Wannier |first= Gregory H. |author-link=Gregory Wannier |year=1987 |orig-year=1966 |publisher=Dover Publications |location=New York |isbn=978-0-486-65401-0 |oclc=15520414 |chapter=Chapter 10-2 }}</ref> <math display="block">\nu_\text{peak} = 2.821 \frac{kT}{h}, </math> where * {{math|''k''}} is the Boltzmann constant, * {{math|''h''}} is the Planck constant, * {{math|''T''}} is temperature (in kelvins).
Substituting the Draper point into this equation produces a frequency of 83 THz, or a wavelength of 3.6 μm, which is well into the infrared and completely invisible to the human eye. One example is a black dwarf, which is a theoretical white dwarf that is cooled to a point so it can no longer produce visible light if the temperature is 797 Kelvin or below. However, the leading edge of the blackbody radiation curve extends, at a small fraction of peak intensity, to the near-infrared and far-red (approximately the range 0.7–1 μm), which are weakly visible as a dull red.<ref>{{cite book |title=Biology: Concepts and Applications |first=Cecie |last=Starr |publisher=Thomson Brooks/Cole |year=2005 |isbn=0-534-46226-X |url=https://books.google.com/books?id=RtSpGV_Pl_0C&pg=PA94 }}</ref>
According to the Stefan–Boltzmann law, a black body at the Draper point emits 23 kW of radiation per square meter, almost exclusively infrared.
== See also == * Incandescence
== References == {{reflist}}
<!--Categories--> Category:Heat transfer Category:Thermodynamics Category:Electromagnetic radiation