Wolfram Alpha:

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Blackbody (Cavity) Radiation and Planck's Hypothesis
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"Blackbody radiation" or "cavity radiation" refers to an object or system which absorbs all
system only, not dependent upon the type of radiation which is incident upon it. The radiated
energy can be considered to be produced by standing wave or resonant modes of the cavity
which is radiating.  The amount of radiation emitted in a given frequency range should be
proportional to the number of modes in that range. The best of classical physics suggested
that all modes had an equal chance of being produced, and that the number of modes went up
proportional to the square of the frequency.

But the predicted continual increase in radiated energy with frequency (dubbed the
"ultraviolet catastrophe") did not happen. In order to explain the frequency distribution of
radiation from a hot cavity (blackbody) Planck hypothesized that the equations of motion for
light are a set of harmonic oscillators, one for each possible frequency and and was able
to derive an approximate mathematical function for the black-body spectrum.  However, Planck
soon realized that his solution was not unique. To prove his theory he had to resort to
statistical mechanics to establish that the energy of the oscillators was discrete rather
than continuous.  Basically, Planck had imposed the quantization of the energy of the
oscillators. Applying this new approach to Wien's displacment law showed that the energy must
be proportional to the frequency of the oscillator, the first version of what is now termed
"Planck's relation":

E = hν = hc/λ  since λν = c

Planck was able to calculate the value of h from experimental data on black-body radiation:
his result, 6.55 x 10-34 J/s, is within 1.2% of the currently accepted value. He
was also able to make the first determination of the Boltzmann constant kB from
the same data and theory.```