The Draper point is the approximate temperature above which almost all solid materials visibly glow as a result of

blackbody radiation Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific, continuous spect ...

. It was established at 977 °F (525 °C, 798 K) by

John William Draper John William Draper (May 5, 1811 – January 4, 1882) was an English-born American scientist, philosopher, physician, chemist, historian and photographer. He is credited with producing the first clear photograph of a female face (1839–40) and ...

in 1847. Bodies at temperatures just below the Draper point radiate primarily in the

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

range and emit negligible visible light. The value of the Draper point can be calculated using

Wien's displacement law Wien's displacement law states that the black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature. The shift of that peak is a direct consequence of the Planck r ...

: the peak

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

\nu_\text

(in

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...

) emitted by a blackbody relates to temperature as follows:

\nu_\text = 2.821 \frac,

where * is

Boltzmann's constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...

, * is Planck's constant, * is temperature (in

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...

s). 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

and completely

invisible Invisibility is the state of an object that cannot be seen. An object in this state is said to be ''invisible'' (literally, "not visible"). The phenomenon is studied by physics and perceptual psychology. Since objects can be seen by light in ...

to the human eye. 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. According to the

Stefan–Boltzmann law The Stefan–Boltzmann law describes the power radiated from a black body in terms of its temperature. Specifically, the Stefan–Boltzmann law states that the total energy radiated per unit surface area of a black body across all wavelengths ...

, a black body at the Draper point emits 23 kW of radiation per square metre, almost exclusively infrared.

References

{{reflist Heat transfer Thermodynamics Electromagnetic radiation

See also

References