Derivation From Planck's Law
Wilhelm Wien first derived this law in 1893 by applying the laws of thermodynamics to electromagnetic radiation. A modern variant of Wien's derivation can be found in the textbook by Wannier.
Planck's law for the spectrum of black body radiation may be used to find the actual constant in the peak displacement law. Specifically, the spectral energy density (that is, the energy density per unit wavelength) is
Differentiating u(λ,T) with respect to λ and setting the derivative equal to zero gives
which can be simplified to give
By defining the dimensionless quantity
then the equation becomes
The numerical solution to this equation is
Solving for the wavelength λ in units of nanometers, and using kelvins for the temperature yields:
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 giving x = 2.82143937212...
Using the value 4 in this equation (midway between 3 and 5) yields a "compromise" wavelength-frequency-neutral peak, which is given for x = 3.92069039487....
Read more about this topic: Wien's Displacement Law
Famous quotes containing the word law:
“The image cannot be dispossessed of a primordial freshness, which idea can never claim. An idea is derivative and tamed. The image is in the natural or wild state, and it has to be discovered there, not put there, obeying its own law and none of ours. We think we can lay hold of image and take it captive, but the docile captive is not the real image but only the idea, which is the image with its character beaten out of it.”
—John Crowe Ransom (1888–1974)