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EFFECT
OF INTENSITY
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| 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. |
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OTHER
FUNDAMENTAL LAWS OF PHOTO ELECTRIC EMISSION
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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. |
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EINSTEIN'S
PHOTOELECTRIC EQUATION
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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. |
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| 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. |
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| 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 |
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| Since minimum amount of energy to remove electron from a surface is equal to work function, we can write Einstein equation as: | ||||
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Energy
Supplied = Energy Consumed in ejecting an electron + maximum Kinetic energy
of electron
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| Equations from (1) to (6) are identical and are known as Einstein's photoelectric equations. | ||||
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