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Wiens law

Black body thermal emission intensity as a function of wavelength for various (absolute) temperatures. Wien's law is not obvious in the picture, because the total emission includes a geometrical factor of 1/λ2 which counts the number of fourier modes of wavelength λ, and a second factor of 1/λ2 to convert intensities per-unit-frequency to intensities per-unit-wavelength

Wien's displacement law states that the wavelength distribution of radiated heat energy 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, or moved over, on the graph. The average heat energy in each mode with frequency \nu only depends on the combination ν/T. Restated in terms of the wavelength λ Template:= c/ν, the distributions at corresponding wavelengths are related, where corresponding wavelengths are at locations proportional to 1/T.

From this general law, it follows that there is an inverse relationship between the wavelength of the peak of the emission of a black body and its temperature when expressed as a function of wavelength (frequency gives a different peak as noted later), and this less powerful consequence is often also called Wien's displacement law in many textbooks.

\lambda_{\mathrm{max}} = \frac{b}{T}

where λmax is the peak wavelength, T is the absolute temperature of the blackbody, and b is a constant of proportionality called Wien's displacement constant, equal to 2.8977685(51)×10−3 m·K (2002 CODATA recommended value)

The two digits between the parentheses denote the uncertainty (the standard deviation at 68.27% confidence level) in the two least significant digits of the mantissa.

For wavelengths near the visible spectrum, it is often more convenient to use the nanometer in place of the meter as the unit of measure. In this case, b Template:= 2,897,768.5(51) nm·K.

In the field of plasma physics, where temperatures are often conveniently measured in units of electron volts, the proportionality constant becomes b Template:= 249.71066 nm·eV.

Explanation and familiar approximate applications Edit

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

Max Planck reinterpreted a constant closely related to Wien's constant b as a new constant of nature, which relates the frequency of light to the energy of a light quantum.

Wien's displacement law implies that the hotter an object is, the shorter the wavelength at which it will emit most of its radiation, and also that the frequency for maximal or peak radiation power is found by dividing Wien's constant by the temperature in kelvins.

Examples:

  • Light from the Sun and Moon. The surface temperature (or more correctly, the effective temperature) of the Sun is 5778 K. Using Wien's law, this temperature corresponds to a peak emission at a wavelength of 2.89777 million nm K/ 5778 K = 502 nm = about 5000 Å. This wavelength is fairly in the middle of the most sensitive part of land animal visual spectrum acuity. Even nocturnal and twilight-hunting animals must sense light from the waning day and from the moon, which is reflected sunlight with this same wavelength distribution. Also, the average wavelength of starlight maximal power is in this region, due to the sun being in the middle of a common temperature range of stars.

[See for example the article color, because of the spread resulting in white light. Due to the Rayleigh scattering of blue light by the atmosphere this white light is separated somewhat, resulting in a blue sky and a yellow sun].

Wien's constant may be used in different units, and many examples to calculate familiar situation types of radiation required use of only one or two significant figures:

  • Light from incandescent bulbs and fires. A lightbulb has a glowing wire with a somewhat lower temperature, resulting in yellow light, and something that is "red hot" is again a little less hot. It is easy to calculate that a wood fire at 1500 K puts out peak radiation at 3 million nm K /1500 K = 2000 nm = 20,000 Å. This is far more energy in the infrared than in the visible band, which ends about 7500 Å.
  • Radiation from mammals and the living human body. Mammals at roughly 300 K emit peak radiation at 3 thousand μm K / 300 K = 10 μm, in the far infrared. This is therefore the range of infrared wavelengths that pit viper snakes and passive IR cameras must sense.
  • The wavelength of radiation from the Big Bang. A typical application of Wien's law would also be to the blackbody radiation resulting from the Big Bang. Remembering that Wien's displacement constant is about 3 mm K, and the temperature of the Big Bang background radiation is about 3 K (actually 2.7 K), it is apparent that the microwave background of the sky peaks in power at 2.9 mm K / 2.7 K = just over 1 mm wavelength in the microwave spectrum. This provides a convenient rule of thumb for why microwave equipment must be sensitive on both sides of this frequency band, in order to do effective research on the cosmic microwave background.

Frequency form Edit

In terms of frequency \nu (in hertz), Wien's displacement law becomes

\nu_{max} = { \alpha \over h} kT  \approx  (5.879 \times 10^{10} \ \mathrm{Hz/K}) \cdot T

where α ≈ 2.821439... is a constant resulting from the numerical solution of the maximization equation, k is the Boltzmann constant, h is the Planck constant, and T is the temperature (in kelvins).

Because the spectrum resulting from Planck's law of black body radiation takes a different shape in the frequency domain from that of the wavelength domain, the frequency location of the peak emission does not correspond to the peak wavelength using the simple relationship between frequency, wavelength, and the speed of light.

Derivation Edit

Wilhelm Wien first derived this law in 1893 by applying the laws of thermodynamics to electromagnetic radiation.[1] A modern variant of Wien's derivation can be found in the textbook by Wannier.[2]

Wien noted that under adiabatic expansion, the energy of a mode of light, the frequency of the mode, and the total temperature of the light change together in the same way, so that their ratios are constant. This implies that in each mode at thermal equilibrium, the adiabatic invariant energy/frequency should only be a function of the adiabatic invariant frequency/temperature:

 u(k,T)/\nu = F(\nu/T)\,.

The form of F is now known from Planck's law:

 F(\nu/T) = {1\over e^{h\nu /kT} -1} \approx e^{-h\nu/kT}\, .

Wien guessed the approximate pure exponential form, which is Wien's distribution law, a valid high frequency approximation to Planck's law. However, no matter what the function F is, the location of the peak of the distribution as a function of frequency is strictly proportional to T.

To get the usual expression for the blackbody curve, the energy per mode needs to be multiplied by the number density of modes with a given frequency \nu:

 d^3k = 4\pi |k|^2 d|k| \propto \nu^2 d\nu

so that this number density is proportional to the frequency squared. The total energy per unit frequency adds the \nu^2 modes together to give the total energy at frequency \nu:

 u(\nu,T) \propto \nu^3 F(\nu/T) .

This per-unit-frequency expression for the density can be transformed to a per-unit-wavelength density by changing variables:

 u(\nu,T) d\nu = u(\lambda,T) d\lambda \,,

and since \nu=c/\lambda, this adds a factor of {d\nu\over d\lambda}= {c\over \lambda^2}:

 u(\lambda,T) \propto {1\over \lambda^5} F(c/\lambda T) \,.

These different variables only introduce a power-law in front of the function F. For any function U of the form:

 U(x) = x^a F(x/T)\,

the location of the maximum or minimum of U is where the derivative is zero:

0= {dU \over dx} = a x^{a-1} F(x/T ) + {x^a \over T} F'(x/T).

Dividing through by x^{a-1}, one is left with the equation:

 a F(x/T) + {x\over T} F'(x/T) =0 ,

which is an equation for x/T, so that the minima or maxima of U are at some definite value of x/T, at an x always strictly proportional to T. This is the peak displacement law: the peak location is proportional to the temperature whether the density is expressed in terms of wavenumber, in terms of frequency, in terms of (1/wavelength), or in terms of any other variable where the intensity only gets multiplied by a power of this variable.

The exact numerical location of the peak of the distribution depends on whether the distribution is considered per-mode-number, per-unit-frequency, or per-unit-wavelength, since the power law in front of F is different for the different forms.

To find the actual constant in the peak displacement law, use Planck's law for the spectrum of black body radiation:

u(\lambda,T) = {8\pi h c\over \lambda^5}{1\over e^{h c/\lambda kT}-1}.

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

{ \partial u \over \partial \lambda } = 8\pi h c\left( {hc\over kT \lambda^7}{e^{h c/\lambda kT}\over \left(e^{h c/\lambda kT}-1\right)^2} -  {1\over\lambda^6}{5\over e^{h c/\lambda kT}-1}\right)=0

which can be simplified to give

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

If we define the dimensionless quantity x to be

x\equiv{hc\over\lambda kT }

then the equation above becomes

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

The numerical solution to this equation is[3]:

x = 4.965114231744276\ldots

Solving for the wavelength \lambda in units of nanometers, and using kelvins for the temperature yields:

\lambda_{max} = {hc\over x }{1\over kT} = {2.89776829\ldots \times 10^6 \ \mathrm{nm \cdot K} \over T}.

The frequency form of Wien's displacement law is derived using similar methods, but starting with Planck's law in terms of frequency instead of wavelength. The effective result is to substitute 3 for 5 in the equation for the peak wavelength. This is solved with x = 2.82143937212...

Using the constant of 4 in this equation (midway between 3 and 5) yields a "compromise" wavelength-frequency-neutral peak, which is given for x = 3.92069039487....

External links Edit

References and notesEdit

  1. Mehra, Jagdish; Rechenberg, Helmut (1982). "Volume 1, Chapter 1". The historical development of quantum theory. New York: Springer-Verlag. ISBN 9780387906423. OCLC 7944997. 
  2. Wannier, Gregory H (1987). "Chapter 10-2". Statistical Physics. New York: Dover Publications. ISBN 9780486654010. OCLC 15520414. 
  3. The equation {x e^{x}\over e^{x} - 1} = n cannot be solved in terms of elementary functions. It can be solved in terms of Lambert's Product Log function but an exact solution is not important in this derivation.
  • B. H. Soffer and D. K. Lynch, "Some paradoxes, errors, and resolutions concerning the spectral optimization of human vision," Am. J. Phys. 67 (11), 946-953 1999.
  • M. A. Heald, "Where is the 'Wien peak'?", Am. J. Phys. 71 (12), 1322-1323 2003.ca:Llei de Wien

cs:Wienův posunovací zákoneo:Leĝo de Wien pri ŝoviĝoit:Legge di Wilhelm Wien he:חוק וין ml:വിയെന്നിന്റെ ഡിസ്പ്ലേസ്മെന്റ് നിയമം nl:Wet van Wienno:Wiens forskyvningslov nn:Wien si forskyvingslov pl:Prawo Wiena pt:Lei de Wien sl:Wienov zakon fi:Wienin siirtymälaki sv:Wiens lag

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