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### William J. Prest says

Einstein started it with his seminal work on the photo electric effect in 1905, where he defined the electromagnetic interaction of certain frequencies on matter. He noted that some frequencies of light would kick electrons out of metals, while most would not. He came to call this effect the "quanta" of the photoelectric effect. He also found that this process was reversible insofar as when the electron connected back to the atom where it had been ejected, a photon, a quanta of electromagnetic radiation of exactly the same frequency as that which kicked it out, was emitted instead. On this discovery and commentary was built all the sum of following quantum theory that describes all atomic and sub atomic processes as discrete phenomenon called quanta, hence quantum theory. It took a few years for physicists to catch up and the first person to approach him was Max Planck who had ideas of quantum mechanics of his own and found the proof he needed in the Photoelectric effect.

Einstein would end up arguing against quantum mechanics and aspects of it that he called "spooky action at a distance". But the findings of quantum mechanics are by and large confirmed. The problem now is to unite general relativity (GR) with quantum theory and so far, no one has successfully done this. Quantum theory has become complex, even unwieldy, but it accurately describes conditions at the atomic and sub atomic levels of reality.

### Deena says

Planck's Quantum Theory of radiation:

According to this theory,radiant energy is emitted or absorbed not continuously but discontinuously in the form of quanta.Each quanta is associated with definite amount of energy.

The energy (E)associated with each quantum foot is a particular radiation of frequency 'v' is given by E=hv where h is planck's constant.

In the case of light,the quantum of energy is called photon.

A body can emit or absorb either one quantum of energy or some whole number multiple of it i.e,hv,2hv,3hv etc called as quantisation of energy.