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Einsteinís Photo-Electric Equation
Einstein used Planckís 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, fo, 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.  
 
   
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 photo-electric equation) suggests that if a graph is plotted of Kmax 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 photo-electric effect allow us to find a value for h (6.63◊10-34Js) and the work function of the metal.  
 
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