Reference: Disturbance Theory
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In his very first paper published in 1905 Einstein establishes the concept of “energy quanta” or “light quanta”. The manifestation of light quantum (photon) becomes more pronounced as the frequency of radiation increases.
Einstein’s original paper with comments
In his paper Einstein analyzes the work done by Wien and Planck on Black Body radiation [see Blackbody radiation (Notes)] and makes the following fundamental observations.

According to Maxwell’s theory, the energy must be considered to be a continuous function in space for all purely electromagnetic phenomena, thus also for light.

According to Kinetic Theory of gases, the energy in a volume can be written as a sum of energy of a finite number of particles localized in space, which move without being divided.

The classical treatment of energy as a continuous function in space fails to predict the energy spectrum observed for the black body radiation.

Max Planck’s postulate, “energy is proportional to the frequency of radiation” completely predicts the energy spectrum observed for the black body radiation.

Planck’s equation provides the classical results for low frequencies, showing that radiation energy is a continuous function in space at low frequencies only.

Using Wein’s law that matches experimental observations of blackbody radiation at high frequencies, Einstein proves the energy behavior of radiation to be particlelike. From this arises Einstein’s proposal of “energy quanta” or “light quanta”.

Einstein’s demonstrates that the use of “statistical probability” by Boltzmann is compatible with the principles of physics, and they can be used to estimate the magnitude of such energy quanta.

The idea of energy quanta is compatible with the observations made in photoluminescence resulting in the Stoke’s Rule.

The calculated magnitude of “energy quanta” is in line with the experimental data obtained from photoelectricity.

In conclusion, the concept of energy quanta is also compatible with ionization of gases by ultraviolet light.
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