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A blackbody is an idealized object which absorbs and emits
all frequencies. Classical physics can be used to derive an equation
which describes the intensity of blackbody radiation as a function of
frequency for a fixed temperaturethe result is known as the
RayleighJeans law. Although the RayleighJeans law works for low
frequencies, it diverges as ;
this divergence for high
frequencies is called the ultraviolet catastrophe.
Max Planck explained the blackbody radiation in 1900 by assuming
that the energies of the oscillations of electrons which gave rise to
the radiation must be proportional to integral multiples of the
frequency, i.e.,

(1) 
Using statistical mechanics, Planck derived an equation similar to the
RayleighJeans equation, but with the adjustable parameter h. Planck
found that for
J s, the experimental data
could be reproduced. Nevertheless, Planck could not offer a good
justification for his assumption of energy quantization. Physicicsts
did not take this energy quantization idea seriously until Einstein
invoked a similar assumption to explain the photoelectric effect.
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