Einstein used Planck’s idea of quantization of electromagnetic
radiation to explain the results of experiments on the
photoelectric 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 photoelectric emission is to occur, the quanta of
electromagnetic radiation must possess at least this quantity of
energy. 

Therefore, the threshold frequency, f_{o}, for
photoemission is given by 





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





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 





This equation (now called Einstein's photoelectric equation) suggests that if a graph is plotted of K_{max}
against f, we will have a straight line. 



This is what is found by experiment. 

Furthermore, the slope of the line is equal to h,
Planck's constant and the intercept on the vertical axis is at w. 

Thus, experiments on the photoelectric effect allow us to find
a value for h (6.63×10^{34}Js) and the work function of the metal. 
