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Wien derived his law from thermodynamic arguments, several years before Planck introduced the quantization of radiation.⁵

Wien's original paper did not contain the Planck constant.⁶ In this paper, Wien took the wavelength of black-body radiation and combined it with the Maxwell–Boltzmann energy distribution for atoms. The exponential curve was created by the use of Euler's number e raised to the power of the temperature multiplied by a constant. Fundamental constants were later introduced by Max Planck.⁷

The law may be written as⁸ I ( ν , T ) = 2 h ν 3 c 2 e − h ν k B T , {\displaystyle I(\nu ,T)={\frac {2h\nu ^{3}}{c^{2}}}e^{-{\frac {h\nu }{k_{\text{B}}T}}},} (note the simple exponential frequency dependence of this approximation) or, by introducing natural Planck units, I ( ν , x ) = 2 ν 3 e − x , {\displaystyle I(\nu ,x)=2\nu ^{3}e^{-x},} where:

• I ( ν , T ) {\displaystyle I(\nu ,T)} is the amount of energy per unit surface area per unit time per unit solid angle per unit frequency emitted at a frequency ν, the so called spectral radiance,
• T {\displaystyle T} is the temperature of the black body,
• x {\displaystyle x} is the ratio of frequency over temperature,
• h {\displaystyle h} is the Planck constant,
• c {\displaystyle c} is the speed of light,
• k B {\displaystyle k_{\text{B}}} is the Boltzmann constant.

This equation may also be written as⁹¹⁰ I ( λ , T ) = 2 h c 2 λ 5 e − h c λ k B T , {\displaystyle I(\lambda ,T)={\frac {2hc^{2}}{\lambda ^{5}}}e^{-{\frac {hc}{\lambda k_{\text{B}}T}}},} where I ( λ , T ) {\displaystyle I(\lambda ,T)} is the amount of energy per unit surface area per unit time per unit solid angle per unit wavelength emitted at a wavelength λ. Wien acknowledges Friedrich Paschen in his original paper as having supplied him with the same formula based on Paschen's experimental observations.¹¹

The peak value of this curve, as determined by setting the derivative of the equation equal to zero and solving,¹² occurs at a wavelength λ max = h c 5 k B T ≈ 0.2878   c m ⋅ K T , {\displaystyle \lambda _{\text{max}}={\frac {hc}{5k_{\text{B}}T}}\approx {\frac {\mathrm {0.2878~cm\cdot K} }{T}},} and frequency ν max = 3 k B T h ≈ 6.25 × 10 10   H z K ⋅ T . {\displaystyle \nu _{\text{max}}={\frac {3k_{\text{B}}T}{h}}\approx \mathrm {6.25\times 10^{10}~{\frac {Hz}{K}}} \cdot T.}

Relation to Planck's law

The Wien approximation was originally proposed as a description of the complete spectrum of thermal radiation, although it failed to accurately describe long-wavelength (low-frequency) emission. However, it was soon superseded by Planck's law, which accurately describes the full spectrum, derived by treating the radiation as a photon gas and accordingly applying Bose–Einstein in place of Maxwell–Boltzmann statistics. Planck's law may be given as¹³ I ( ν , T ) = 2 h ν 3 c 2 1 e h ν k T − 1 . {\displaystyle I(\nu ,T)={\frac {2h\nu ^{3}}{c^{2}}}{\frac {1}{e^{\frac {h\nu }{kT}}-1}}.}

The Wien approximation may be derived from Planck's law by assuming h ν ≫ k T {\displaystyle h\nu \gg kT} . When this is true, then¹⁴ 1 e h ν k T − 1 ≈ e − h ν k T , {\displaystyle {\frac {1}{e^{\frac {h\nu }{kT}}-1}}\approx e^{-{\frac {h\nu }{kT}}},} and so the Wien approximation gets ever closer to Planck's law as the frequency increases.

Other approximations of thermal radiation

The Rayleigh–Jeans law developed by Lord Rayleigh may be used to accurately describe the long wavelength spectrum of thermal radiation but fails to describe the short wavelength spectrum of thermal emission.¹⁵¹⁶

See also

• ASTM Subcommittee E20.02 on Radiation Thermometry
• Sakuma–Hattori equation
• Ultraviolet catastrophe
• Wien's displacement law

References

1. Wien, W. (1897). "On the division of energy in the emission-spectrum of a black body" (PDF). Philosophical Magazine. Series 5. 43 (262): 214–220. doi:10.1080/14786449708620983. http://myweb.rz.uni-augsburg.de/~eckern/adp/history/historic-papers/1896_294_662-669.pdf ↩

2. Mehra, J.; Rechenberg, H. (1982). The Historical Development of Quantum Theory. Vol. 1. Springer-Verlag. Chapter 1. ISBN 978-0-387-90642-3. 978-0-387-90642-3 ↩

3. Bowley, R.; Sánchez, M. (1999). Introductory Statistical Mechanics (2nd ed.). Clarendon Press. ISBN 978-0-19-850576-1. 978-0-19-850576-1 ↩

4. Bowley, R.; Sánchez, M. (1999). Introductory Statistical Mechanics (2nd ed.). Clarendon Press. ISBN 978-0-19-850576-1. 978-0-19-850576-1 ↩

5. Wien, W. (1897). "On the division of energy in the emission-spectrum of a black body" (PDF). Philosophical Magazine. Series 5. 43 (262): 214–220. doi:10.1080/14786449708620983. http://myweb.rz.uni-augsburg.de/~eckern/adp/history/historic-papers/1896_294_662-669.pdf ↩

6. Wien, W. (1897). "On the division of energy in the emission-spectrum of a black body" (PDF). Philosophical Magazine. Series 5. 43 (262): 214–220. doi:10.1080/14786449708620983. http://myweb.rz.uni-augsburg.de/~eckern/adp/history/historic-papers/1896_294_662-669.pdf ↩

7. Crepeau, J. (2009). "A Brief History of the T4 Radiation Law". ASME 2009 Heat Transfer Summer Conference. Vol. 1. ASME. pp. 59–65. doi:10.1115/HT2009-88060. ISBN 978-0-7918-4356-7. 978-0-7918-4356-7 ↩

8. Rybicki, G. B.; Lightman, A. P. (1979). Radiative Processes in Astrophysics. John Wiley & Sons. ISBN 978-0-471-82759-7. 978-0-471-82759-7 ↩

9. Bowley, R.; Sánchez, M. (1999). Introductory Statistical Mechanics (2nd ed.). Clarendon Press. ISBN 978-0-19-850576-1. 978-0-19-850576-1 ↩

10. Modest, M. F. (2013). Radiative Heat Transfer. Academic Press. pp. 9, 15. ISBN 978-0-12-386944-9. 978-0-12-386944-9 ↩

11. Wien, W. (1897). "On the division of energy in the emission-spectrum of a black body" (PDF). Philosophical Magazine. Series 5. 43 (262): 214–220. doi:10.1080/14786449708620983. http://myweb.rz.uni-augsburg.de/~eckern/adp/history/historic-papers/1896_294_662-669.pdf ↩

12. Irwin, J. A. (2007). Astrophysics: Decoding the Cosmos. John Wiley & Sons. p. 130. ISBN 978-0-470-01306-9. 978-0-470-01306-9 ↩

13. Rybicki, G. B.; Lightman, A. P. (1979). Radiative Processes in Astrophysics. John Wiley & Sons. ISBN 978-0-471-82759-7. 978-0-471-82759-7 ↩

14. Rybicki, G. B.; Lightman, A. P. (1979). Radiative Processes in Astrophysics. John Wiley & Sons. ISBN 978-0-471-82759-7. 978-0-471-82759-7 ↩

15. Bowley, R.; Sánchez, M. (1999). Introductory Statistical Mechanics (2nd ed.). Clarendon Press. ISBN 978-0-19-850576-1. 978-0-19-850576-1 ↩

16. Rybicki, G. B.; Lightman, A. P. (1979). Radiative Processes in Astrophysics. John Wiley & Sons. ISBN 978-0-471-82759-7. 978-0-471-82759-7 ↩