Draper point

Draper point

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The Draper point is the approximate temperature above which almost all solid materials visibly glow as a result of blackbody radiation. It was established at 977˚ F (525˚ C, 798 K) by John William Draper
John William Draper
John William Draper was an American scientist, philosopher, physician, chemist, historian, and photographer. He is credited with producing the first clear photograph of a female face and the first detailed photograph of the Moon...

 in 1847.

Bodies at temperatures just below the Draper point radiate primarily in the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 band and at longer wavelengths, and emit negligible visible light. The value of the Draper point can be calculated using Wien's displacement law
Wien's displacement law
Wien's displacement law states that the wavelength distribution of thermal radiation from a black body at any temperature has essentially the same shape as the distribution at any other temperature, except that each wavelength is displaced on the graph...

: the peak frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

  (in hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

) emitted by a blackbody relates to temperature as follows:


where
k is Boltzmann's constant.
h is Planck's constant.
T is temperature (in kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s).


Substituting the Draper point into this equation produces a frequency of 46.9 terahertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

—which is infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

, and largely invisible except for a dull red, at a small fraction of the peak intensity, that is visible to the human eye.