Categories: Physics.

**Planck’s law** describes the radiation spectrum of a **black body**: a theoretical object in thermal equilibrium, which absorbs photons, re-radiates them, and then re-absorbs them.

Since the photon population varies with time, this is a grand canonical ensemble, and photons are bosons (see Pauli exclusion principle), this system must obey the Bose-Einstein distribution, with a chemical potential \(\mu = 0\) (due to the freely varying population):

\[\begin{aligned} f_B(E) = \frac{1}{\exp\!(\beta E) - 1} \end{aligned}\]

Each photon has an energy \(E = \hbar \omega = \hbar c k\), so the density of states is as follows in 3D:

\[\begin{aligned} g(E) = 2 \frac{g(k)}{E'(k)} = \frac{V k^2}{\pi^2 \hbar c} = \frac{V E^2}{\pi^2 \hbar^3 c^3} = \frac{8 \pi V E^2}{h^3 c^3} \end{aligned}\]

Where the factor of \(2\) accounts for the photon’s polarization degeneracy. We thus expect that the number of photons \(N(E)\) with an energy between \(E\) and \(E + \dd{E}\) is given by:

\[\begin{aligned} N(E) \dd{E} = f_B(E) \: g(E) \dd{E} = \frac{8 \pi V}{h^3 c^3} \frac{E^2}{\exp\!(\beta E) - 1} \dd{E} \end{aligned}\]

By substituting \(E = h \nu\), we find that the number of photons \(N(\nu)\) with a frequency between \(\nu\) and \(\nu + \dd{\nu}\) must be as follows:

\[\begin{aligned} N(\nu) \dd{\nu} = \frac{8 \pi V}{c^3} \frac{\nu^2}{\exp\!(\beta h \nu) - 1} \dd{\nu} \end{aligned}\]

Multiplying by the energy \(h \nu\) yields the distribution of the radiated energy, which we divide by the volume \(V\) to get Planck’s law, also called the **Plank distribution**, describing a black body’s radiated spectral energy density per unit volume:

\[\begin{aligned} \boxed{ u(\nu) = \frac{8 \pi h}{c^3} \frac{\nu^3}{\exp\!(\beta h \nu) - 1} } \end{aligned}\]

The Planck distribution peaks at a particular frequency \(\nu_{\mathrm{max}}\), which can be found by solving the following equation for \(\nu\):

\[\begin{aligned} 0 = u'(\nu) \quad \implies \quad 0 = 3 \nu^2 (\exp\!(\beta h \nu) - 1) - \nu^3 \beta h \exp\!(\beta h \nu) \end{aligned}\]

By defining \(x \equiv \beta h \nu_{\mathrm{max}}\), this turns into the following transcendental equation:

\[\begin{aligned} 3 = (3 - x) \exp\!(x) \end{aligned}\]

Whose numerical solution leads to **Wien’s displacement law**, given by:

\[\begin{aligned} \boxed{ \frac{h \nu_{\mathrm{max}}}{k_B T} \approx 2.822 } \end{aligned}\]

Which states that the peak frequency \(\nu_{\mathrm{max}}\) is proportional to the temperature \(T\).

Because \(u(\nu)\) represents the radiated spectral energy density, we can find the total radiated energy \(U\) per unit volume by integrating over \(\nu\):

\[\begin{aligned} U &= \int_0^\infty u(\nu) \dd{\nu} = \frac{8 \pi h}{c^3} \int_0^\infty \frac{\nu^3}{\exp\!(\beta h \nu) - 1} \dd{\nu} \\ &= \frac{8 \pi h}{\beta^3 h^3 c^3} \int_0^\infty \frac{(\beta h \nu)^3}{\exp\!(\beta h \nu) - 1} \dd{\nu} = \frac{8 \pi}{\beta^4 h^3 c^3} \int_0^\infty \frac{x^3}{\exp\!(x) - 1} \dd{x} \end{aligned}\]

This definite integral turns out to be \(\pi^4/15\), leading us to the **Stefan-Boltzmann law**, which states that the radiated energy is proportional to \(T^4\):

\[\begin{aligned} \boxed{ U = \frac{4 \sigma}{c} T^4 } \end{aligned}\]

Where \(\sigma\) is the **Stefan-Boltzmann constant**, which is defined as follows:

\[\begin{aligned} \sigma \equiv \frac{2 \pi^5 k_B^4}{15 c^2 h^3} \end{aligned}\]

- H. Gould, J. Tobochnik,
*Statistical and thermal physics*, 2nd edition, Princeton.

© Marcus R.A. Newman, a.k.a. "Prefetch".
Available under CC BY-SA 4.0.