David W. Ball

Blackbody radiation, spectroscopy (especially of hydrogen), the photoelectric effect, lowtemperature heat capacities — these
phenomena were a puzzle to scientists of the 19th century. Wellentrenched theories of nature, including Newton's laws of
motion and Maxwell's laws of electrodynamics, which were so successful in understanding the behavior of matter and radiation,
did not help explain these behaviors completely.
As I've said before, if there is a disagreement between theory and nature, we've got to change either nature or theory. Because
every attempt to change the universe has failed, our only choice is to get a new theory. That's what eventually happened,
and it started with a thermodynamicist.
Max Planck's Quantum Theory
Figure 1

Max Planck (1858–1947) was a German theoretical physicist who was trained in thermodynamics. This was fortunate, for when
Planck began to consider the issue involving blackbody radiation, he realized that Wien's law (1) would apply only if the
entropy of the light depended upon its energy. Planck realized that if this were true in the highfrequency portion of the
electromagnetic spectrum (where Wien's law applied), it had to be true in the lowfrequency portion of the spectrum, where
the RayleighJeans law was approximately correct.

Thus, Planck sought to combine the two laws into one mathematical framework. He was able to do this and derived a formula
that did predict the nature of blackbody radiation — but he realized that this mathematical expression had to have some physical
justification. So Planck looked into what physical basis was needed to justify his equation. Apparently, he did not like the
conclusions he arrived at: that entropy was a statistical concept, not a deterministic or absolute one. In addition, Planck
had to assume that the vibrating atoms in the blackbody could not absorb energy continuously, but only in certain amounts
that were proportional to their vibrational frequency ν, not their amplitude A (Figure 1):
To make this proportionality an equality, a proportionality constant is needed:
where h is now known as Planck's constant.
The formula for the intensity of blackbody radiation that Planck derived was
where h is Planck's constant, c is the speed of light, k is Boltzmann's constant, and T is the absolute temperature. This equation is known by several names: Planck's law, the radiation distribution law, the law
of blackbody radiation, and so forth. In terms of wavelength λ, Planck's law is