Einstein, black-body radiation photoelectric effect

In 1900 Max Planck proposed a solution to the problem of black-body radiation described above. He suggested that when electromagnetic radiation interacts with matter, energy can only be absorbed or emitted in certain discrete amounts, called quanta. Planck s theory will not be described here, as it is highly technical. In any case, Planck s proposal was timid compared with the theory that followed. He supposed that quanta were only important in absorption and emission of radiation, but that otherwise the wave theory did not need to be modified. It was Einstein who took a more radical step in 1905 (the year in which he published his first paper on the theory of relativity and on several other unrelated topics). Einstein s analysis of the photoelectric effect is crucial, and has led to a complete change in the way we think of light and other radiation. 

Although the corpuscular aspect of electromagnetic radiation, which was surmised by Newton in the seventeenth century, was used by Planck in 1900 to explain Wien s black body radiation law and by Einstein in 1905 to explain Lenard s photoelectric effect, its most spectacular demonstration was Compton s explanation in 1923 of the anomalous scattering of X-rays by bound electrons. 

The old quantum theory includes Planck s black-body radiation theory, Einstein s theory of the photoelectric effect, and Bohr s theory of the hydrogen atom. 

The old quantum theory consists of theories with arbitrary assumptions of quantization that were devised to explain phenomena that classical physics could not explain. The old quantum theory includes the black-body radiation theory of Planck, the photoelectric effect theory of Einstein, and the hydrogen atom theory of Bohr. 

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