Quantum Theory: Albert Einstein
The Wave Structure of Matter and Standing Wave Interactions (which only occur at discrete Frequencies) explains the Quantum Energy States of Matter and Light 'Quanta' (Photoelectric Effect) as formalised by Albert Einstein in 1905

The more success the quantum mechanics has, the sillier it looks.
(Albert Einstein to Heinrich Zangger on Quantum Theory, May 20, 1912)

Quantum Theory: Albert Einstein, Theoretical Physicist / Philosopher of Science (1879 - 1955)

Albert Einstein's work on the Photoelectric effect relates to Max Planck's discovery that light energy is emitted and absorbed in discrete quanta of energy (contrary to predictions of Maxwell's equations and the continuous electromagnetic theory of light). As Einstein explains;

In the year nineteen hundred, in the course of purely theoretical (mathematical) investigation, Max Planck made a very remarkable discovery: the law of radiation of bodies as a function of temperature could not be derived solely from the Laws of Maxwellian electrodynamics. To arrive at results consistent with the relevant experiments, radiation of a given frequency f had to be treated as though it consisted of energy atoms (photons) of the individual energy hf, where h is Planck's universal constant.
During the years following, it was shown that light was everywhere produced and absorbed in such energy quanta. In particular, Niels Bohr was able to largely understand the structure of the atom, on the assumption that the atoms can only have discrete energy values, and that the discontinuous transitions between them are connected with the emission or absorption of energy quantum. This threw some light on the fact that in their gaseous state elements and their compounds radiate and absorb only light of certain sharply defined frequencies. (Albert Einstein, 1940)

Even the Greeks had already conceived the atomistic nature of matter and the concept was raised to a high degree of probability by the scientists of the nineteenth century. But it was Planck's law of radiation that yielded the first exact determination - independent of other assumptions - of the absolute magnitudes of atoms. More than that, he showed convincingly that in addition to the atomistic structure of matter there is a kind of atomistic structure to energy, governed by the universal constant h, which was introduced by Planck.
This discovery became the basis of all twentieth-century research in physics and has almost entirely conditioned its development ever since. Without this discovery it would not have been possible to establish a workable theory of molecules and atoms and the energy processes that govern their transformations. Moreover, it has shattered the whole framework of classical mechanics and electrodynamics and set science a fresh task: that of finding a new conceptual basis for all physics. Despite remarkable partial gains, the problem is still far from a satisfactory solution. (Albert Einstein, 1950)

Albert Einstein (1905) used Planck's relationship to explain the results of the photoelectric effect which showed that the energy E of ejected electrons was dependent upon the frequency f of incident light as described in the equation E=hf. It is ironic that in 1921 Albert Einstein was awarded the Nobel Prize for this discovery, though he never believed in particles and acknowledged that he did not know the cause of the discrete energy transfers (photons) which were contradictory to his continuous field theory of matter!
In 1954 Albert Einstein wrote to his friend Michael Besso expressing his frustration;Quantum-Physics