Hypothesis
of "quanta", introduced by Planck, was found useful
by Einstein for explanation of the photoelectric effect and for
studies of hydrogen spectra. Almost, almost, without Wave
Mechanics
The begining of Modern Physics
is usually attributed to the lecture of Max Planck done on December 14th,
1900 (and on October 19th, 1900) at the meeting of German Physical Society
[1]. In these lectures he formulated the hypothesis that light brings energy
in packets (he called them "Energieelement").
Planck did not use the term “quant”, althrough
he knew this word, what is clear from his article just next page after the
work [1]. Even in 1913 on the “Solvay” Congress he continued to insist that
"photon" is a wrong hipothesis.
At that time, Wave Mechanics was still
to be formulated: first by deBroglie's hypothesis, then in Schrödinger
equation, Dirac's equation and so on.
But just the merely idea of quantisation of physical variables allowed
Niels Bohr to construct his model of Hydrogen atom,
Einstein to predict laser, Pauli to explain
Mendeleev table and so on.
[1] M. Planck, Ueber das Gesetz der Energieverteilung im Normalspektrum,
Ann. der Physik IV, 4 (1900) 553
