A history of Quantum Mechanics
It is hard to realise that the electron was only discovered a little over 100 years ago in 1897. That it was not expected is illustrated by a remark made by J J Thomson, the discoverer of the electron. He said
I was told long afterwards by a distinguished physicist who had been present at my lecture that he thought I had been pulling their leg.
The neutron was not discovered until 1932 so it is against this background that we trace the beginnings of quantum theory back to 1859.
In 1905
Einstein examined the photoelectric effect. The photoelectric effect is the release of electrons from certain metals or semiconductors by the action of light. The electromagnetic theory of light gives results at odds with experimental evidence.
Einstein proposed a quantum theory of light to solve the difficulty and then he realised that
Planck's theory made implicit use of the light quantum hypothesis.
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