Einstein used Planck’s idea of quantization of electro-magnetic radiation to explain the results of experiments on the photo-electric effect.

A certain minimum quantity of energy is needed to "liberate" an electron from a metal. This quantity of energy is called the work function, w, of the metal.

If photo-electric emission is to occur, the quanta of electro-magnetic radiation must possess at least this quantity of energy. Therefore, the threshold frequency, f_o, for photo-emission is given by

If quanta (photons) of higher energy are available, the extra energy appears as kinetic energy, of the emitted electron.

So,

Since w represents the minimum quantity of energy needed to remove an electron from a given metal, then the above equation will give us the maximum kinetic energy of emitted electrons by a given photon. Therefore, we can write

If a graph is plotted of K.E._max against f, we will have a straight line of slope h.

This agrees with the results of experiments on the photo-electric effect and can therefore be considered as evidence to support quantum theory. It also gives us a method for finding the value of Planck’s constant (found to be 6·63×10^-34Js).

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