Wien's displacement law

Wien's displacement law is relevant to some everyday experiences:

• A piece of metal heated by a blow torch first becomes "red hot" as the very longest visible wavelengths appear red, then becomes more orange-red as the temperature is increased, and at very high temperatures would be described as "white hot" as shorter and shorter wavelengths come to predominate the black body emission spectrum. Before it had even reached the red hot temperature, the thermal emission was mainly at longer infrared wavelengths, which are not visible; nevertheless, that radiation could be felt as it warms one's nearby skin.
• One easily observes changes in the color of an incandescent light bulb (which produces light through thermal radiation) as the temperature of its filament is varied by a light dimmer. As the light is dimmed and the filament temperature decreases, the distribution of color shifts toward longer wavelengths and the light appears redder, as well as dimmer.
• A wood fire at 1500 K puts out peak radiation at about 2000 nanometers. 98% of its radiation is at wavelengths longer than 1000 nm, and only a tiny proportion at visible wavelengths (390–700 nanometers). Consequently, a campfire can keep one warm but is a poor source of visible light.
• The effective temperature of the Sun is 5778 Kelvin. Using Wien's law, one finds a peak emission per nanometer (of wavelength) at a wavelength of about 500 nm, in the green portion of the spectrum near the peak sensitivity of the human eye.[3][4] On the other hand, in terms of power per unit optical frequency, the Sun's peak emission is at 343 THz or a wavelength of 883 nm in the near infrared. In terms of power per percentage bandwidth, the peak is at about 635 nm, a red wavelength. Regardless of how one wants to plot the spectrum, about half of the sun's radiation is at wavelengths shorter than 710 nm, about the limit of the human vision. Of that, about 12% is at wavelengths shorter than 400 nm, ultraviolet wavelengths, which is invisible to an unaided human eye. It can be appreciated that a rather large amount of the Sun's radiation falls in the fairly small visible spectrum.

The color of a star is determined by its temperature, according to Wien's law. In the constellation of Orion, one can compare Betelgeuse (T ≈ 3300 K, upper left), Rigel (T = 12100 K, bottom right), Bellatrix (T = 22000 K, upper right), and Mintaka (T = 31800 K, rightmost of the 3 "belt stars" in the middle).

• The preponderance of emission in the visible range, however, is not the case in most stars. The hot supergiant Rigel emits 60% of its light in the ultraviolet, while the cool supergiant Betelgeuse emits 85% of its light at infrared wavelengths. With both stars prominent in the constellation of Orion, one can easily appreciate the color difference between the blue-white Rigel (T = 12100 K) and the red Betelgeuse (T ≈ 3300 K). While few stars are as hot as Rigel, stars cooler than the sun or even as cool as Betelgeuse are very commonplace.
• Mammals with a skin temperature of about 300 K emit peak radiation at around 10 μm in the far infrared. This is therefore the range of infrared wavelengths that pit viper snakes and passive IR cameras must sense.
• When comparing the apparent color of lighting sources (including fluorescent lights, LED lighting, computer monitors, and photoflash), it is customary to cite the color temperature. Although the spectra of such lights are not accurately described by the black-body radiation curve, a color temperature (the correlated color temperature) is quoted for which black-body radiation would most closely match the subjective color of that source. For instance, the blue-white fluorescent light sometimes used in an office may have a color temperature of 6500 K, whereas the reddish tint of a dimmed incandescent light may have a color temperature (and an actual filament temperature) of 2000 K. Note that the informal description of the former (bluish) color as "cool" and the latter (reddish) as "warm" is exactly opposite the actual temperature change involved in black-body radiation.

The law is named for Wilhelm Wien, who derived it in 1893 based on a thermodynamic argument.[5] Wien considered adiabatic expansion of a cavity containing waves of light in thermal equilibrium. Using Doppler's principle, he showed that, under slow expansion or contraction, the energy of light reflecting off the walls changes in exactly the same way as the frequency. A general principle of thermodynamics is that a thermal equilibrium state, when expanded very slowly, stays in thermal equilibrium.

Wien himself deduced this law theoretically in 1893, following Boltzmann’s thermodynamic reasoning. It had previously been observed, at least semi-quantitatively, by an American astronomer, Langley. This upward shift in ${\displaystyle \nu _{\mathrm {max} }}$ with T is familiar to everyone—when an iron is heated in a fire, the first visible radiation (at around 900 K) is deep red, the lowest frequency visible light. Further increase in T causes the color to change to orange then yellow, and finally blue at very high temperatures (10,000 K or more) for which the peak in radiation intensity has moved beyond the visible into the ultraviolet.[6]

The adiabatic principle allowed Wien to conclude that for each mode, the adiabatic invariant energy/frequency is only a function of the other adiabatic invariant, the frequency/temperature. From this, he derived the "strong version" of Wien's displacement law: the statement that the blackbody spectral radiance is proportional to ${\displaystyle \nu ^{3}F(\nu /T)}$ for some function F of a single variable. A modern variant of Wien's derivation can be found in the textbook by Wannier[7] and in a paper by E. Buckingham[8]

The consequence is that the shape of the black-body radiation function (which was not yet understood) would shift proportionally in frequency (or inversely proportionally in wavelength) with temperature. When Max Planck later formulated the correct black-body radiation function it did not explicitly include Wien's constant b. Rather, the Planck constant h was created and introduced into his new formula. From the Planck constant h and the Boltzmann constant k, Wien's constant b can be obtained.

For spectral flux considered per unit frequency ${\displaystyle d\nu }$ (in hertz), Wien's displacement law describes a peak emission at the optical frequency ${\displaystyle \nu _{\text{peak}}}$ given by:[9]

${\displaystyle \nu _{\text{peak}}={\alpha \over h}kT\approx (5.879\times 10^{10}\ \mathrm {Hz/K} )\cdot T}$

or equivalently

${\displaystyle h\nu _{\text{peak}}=\alpha kT\approx (2.431\times 10^{-4}\ \mathrm {eV/K} )\cdot T}$

where α ≈ 2.821439372122078893...[10] is a constant resulting from the maximization equation, k is the Boltzmann constant, h is the Planck constant, and T is the temperature (in kelvins). With the emission now considered per unit frequency, this peak now corresponds to a wavelength about 76% longer than the peak considered per unit wavelength. The relevant math is detailed in the next section.

Planck's law for the spectrum of black body radiation predicts the Wien displacement law and may be used to numerically evaluate the constant relating temperature and the peak parameter value for any particular parameterization. Commonly a wavelength parameterization is used and in that case the black body spectral radiance (power per emitting area per solid angle) is:

${\displaystyle u_{\lambda }(\lambda ,T)={2hc^{2} \over \lambda ^{5}}{1 \over e^{hc/\lambda kT}-1}.}$

Differentiating u(λ,T) with respect to λ and setting the derivative equal to zero gives:

${\displaystyle {\partial u \over \partial \lambda }=2hc^{2}\left({hc \over kT\lambda ^{7}}{e^{hc/\lambda kT} \over \left(e^{hc/\lambda kT}-1\right)^{2}}-{1 \over \lambda ^{6}}{5 \over e^{hc/\lambda kT}-1}\right)=0,}$

which can be simplified to give:

${\displaystyle {hc \over \lambda kT}{e^{hc/\lambda kT} \over e^{hc/\lambda kT}-1}-5=0.}$

By defining:

${\displaystyle x\equiv {hc \over \lambda kT},}$

the equation becomes one in the single variable x:

${\displaystyle {xe^{x} \over e^{x}-1}-5=0.}$

which is equivalent to:

${\displaystyle (x-5)e^{x}+5=0.}$

This equation is solved by

${\displaystyle x=5+W_{0}(-5e^{-5})}$

where ${\displaystyle W_{0}}$ is the principal branch of the Lambert W function, and gives x = 4.965114231744276304...[11] to double precision floating point accuracy. Solving for the wavelength λ in millimetres, and using kelvins for the temperature yields:

λ_peak = hc / xkT = (2.897771955185172661... mm⋅K) / T.[12][2]

• Wien approximation
• Emissivity
• Sakuma–Hattori equation
• Stefan–Boltzmann law
• Thermometer
• Ultraviolet catastrophe

• Soffer, B. H.; Lynch, D. K. (1999). "Some paradoxes, errors, and resolutions concerning the spectral optimization of human vision". American Journal of Physics. 67 (11): 946–953. Bibcode:1999AmJPh..67..946S. doi:10.1119/1.19170. S2CID 16025855.
• Heald, M. A. (2003). "Where is the 'Wien peak'?". American Journal of Physics. 71 (12): 1322–1323. Bibcode:2003AmJPh..71.1322H. doi:10.1119/1.1604387.

• Eric Weisstein's World of Physics