The wavelength is related to the frequency by:
The law is sometimes written in terms of the spectral energy density
which has units of energy per unit volume per unit frequency (joule per cubic meter per hertz). Integrated over frequency, this expression yields the total energy density. The radiation field of a black body may be thought of as a photon gas, in which case this energy density would be one of the thermodynamic parameters of that gas.
The spectral energy density can also be expressed as a function of wavelength:
as shown in the derivation below.
Max<unk>  originally produced this law in 1900 (published in 1901) in an attempt to improve upon the Wien approximation, published in 1896 by Wilhelm Wien, which fit the experimental data at short wavelengths (high frequencies) but deviated from it at long wavelengths (low frequencies). The Rayleigh-Jeans law (first published in incomplete form by Rayleigh in 1900) fit well in the complementary domain (long wavelength, low frequency).<unk>  found that the above function,<unk> 's function, fitted the data for all wavelengths remarkably well. 
In constructing a derivation of this law, he considered the possible ways of distributing electromagnetic energy over the different modes of charged oscillators in matter.<unk>  emerged when he assumed that the energy of these oscillators was limited to a set of discrete, integer multiples of a fundamental unit of energy, E, proportional to the oscillation frequency ν:<unk>  made this quantization assumption five years before Albert Einstein hypothesized the existence of photons as a means of explaining the photoelectric effect.  At the time,<unk>  believed that the quantization applied only to the tiny oscillators that were thought to exist in the walls of the cavity (what we now know to be atoms), and made no assumption that light itself propagates in discrete bundles or packets of energy.  Moreover,<unk>  did not attribute any physical significance to this assumption, but rather believed that it was merely a mathematical device that enabled him to derive a single expression for the black body spectrum that matched the empirical data at all wavelengths.
Although<unk> 's formula predicts that a black body will radiate energy at all frequencies, the formula is only practically applicable when many photons are being measured. For example, a black body at room temperature (300 kelvin) with one square meter of surface area  will emit a photon in the visible range once about every thousand years or so, meaning that for most practical purposes, a black body at room temperature does not emit in the visible range. Significance of this fact for the derivation of<unk>  from experimental data, and for the substantiation of the law by the data, is discussed in 
Ultimately,<unk> 's assumption of energy quantization and Einstein's photon hypothesis became the fundamental basis for the development of quantum mechanics.
Planck's law
Hy's law
Franck-Condon principle
Cassie's law
Weber–Fechner law
Boyle's law
Second law of thermodynamics
Equation of state