The Photoelectric Effect (ii)  
 
Light color not intensity determines if electrons
are freed from a metalic surface or not. Einstein
won a Nobel Prize for his explanation of this
phenomena based on the postulate that light arrives
in packets with energy h.f per packet. Where f is the
light frequency and h is Planck's constant.
 
In 1905 Einstein suggested an explanation for Lenard's results. He proposed that light was made up of packets, which we now refer to as photons. He theorized that each packet contained a discrete amount of energy which increased directly with frequency of light in the following way.
E = h.f
Where f is the frequency of the light and h is Planck's constant.
He further suggested that the electrons were bound to the metal in quantum energy states such that it required a specific quantum of energy to free a single electron from the metal surface.
When the quantum of energy, h.f contained in the photon was equal to, or greater than, the quantum of energy reuired to free an electron, then electrons would be freed and the current would flow.
When the quantum
of energy, h.f contained in the photon was less than the quantum of energy required to free an electron from the surface then no electrons would flow.
In Einstein's model more light intensity causes more current to flow, but once the frequency of light falls below the level where a single light packet has sufficient energy to free a single electron ... then no current flows .., regardless of how intense the illumination at that frequency.
Einstein's model predicted that a plot of the stopping voltage Vo versus the frequency of the impinging radiation would be a straight line of slope h/e, where e is the charge on an electron and h is Planck's constant.
Future experimental measurements supported Einstein's model of light as being contained in quantized packets with energy h.f .
