If we draw the photo electric curve by plotting the photo electric current 'I' verses the accelerating voltage 'V', the graph so obtained is shown below. Graph shows that there is a saturation current for different intensities and even when V=0, there is some photo electric current io. The curve shows that the stopping potential is independent of the intensity of radiation.
If these curves are plotted for different frequencies V1 and V2 but with same intensity (graph on right), the curve shows the behavior as shown:
* The saturation current depends upon intensity and not on frequency.
However, the stopping potential becomes more negative from (Vo)1 to (Vo)2 with the increase in frequency.
The no. of electrons emitted per second i.e. photo current is proportional to the intensity of incident light.
If frequency of incident radiation is below threshold frequency, no photo electric emission will take place.
The max. velocity or max. K.E of photoelectrons depends on the frequency of radiation not on intensity. K.E. Increases with the increase in frequency.
The rate at which the electrons are emitted from a photo cathode is independent of its temperature.
This shows that photo electric effect is entirely different from thermionic emission.
For a given metal surface, stopping potential (Vo) is directly proportional to frequency but independent of intensity.
According to Plank's quantum theory, light is emitted from a source in the forms of bundles of energy called photons. Energy of each photon is .
Einstein made use of this theory to explain how photo electric emission takes place.
  According to Einstein, when photons of energy fall on a metal surface, they transfer their energy to the electrons of metal. When the energy of photon is larger than the minimum energy required by the electrons to leave the metal surface, the emission of electrons take place instantaneously.
He proposed that an electron absorbs one whole photon or none. The chance that an electron may absorb more then one electron is negligible because the number of photons is much lower than the electron. After absorbing the photon, an electron either leaves the surface or dissipates its energy within the metal in such a short interval that it has almost no chance to absorb second photon. An increase in intensity of light source simply increases the number of photon and the number of photo electrons but no increase in the energy of photo electron. However, increase in frequency increases the energy of photons and photo electrons.
  According to Einstein's explanation of photoelectric emission, a photon of energy 'E' performs two operations:
1. Removes the electron from the surface of metal
2. Supplies some part of energy to move photo electron towards anode
  Since minimum amount of energy to remove electron from a surface is equal to work function, we can write Einstein equation as:
Energy Supplied = Energy Consumed in ejecting an electron + maximum Kinetic energy of electron
  Equations from (1) to (6) are identical and are known as Einstein's photoelectric equations.
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