When a light radiation of certain minimum frequency known as threshold frequency falls on a certain alkali metal surfaces it has been observed that the electrons get ejected (example working of photovoltaic cell). Hertz working systematically on the photoelectric effect by placing alkali metals in a vaccum tube and noting the current created in the vaccum tube to avoid the influence of the surrounding on the experiment achieved the following results. Here in are also presented the explanation how these results lead to the concept of quantization of the energy associated with waves, here after called as photon and their behavior while interacting with the matter as if they are particles but not waves.
a) Results obtained by varying the frequency of the incident light.
As the frequency increased from a minimum value gradually it was found that below a certain frequency not a single electron was liberated and above this frequency the electrons were liberated. This frequency was named as threshold frequency. This threshold frequency is different for different metals. It is also found that increase in the frequency of the incident light increases the energy of the ejected electrons while their number remained the same. This indicates the transfer of light energy to the electrons in one to one corresponding manner leading to think that the energy associated with each light wave as quanta of energy referred earlier as photon. This one to one interaction is obviously similar to the transfer of momentum from particle to particle involving elastic collision. No release of electron below the threshold frequency indicates the quanta of energy associated with photon is not sufficient to liberate the electron.
b) Variation of intensity of the light below the threshold frequency.
The experiment was also conducted by varying the intensity of light below the threshold frequency. Under, no intensity of light it is found that the electrons are liberated showing that they are not collectively been able to create an energy which is sufficient to knock out a single electron from the metal surface. This non collective effect of light waves clearly indicates the energy transfer is not in favor of wave theory but is in favor of particle theory.
c) Variation of intensity of the light above the threshold frequency.
Above the threshold frequency it has been found that the number of electrons liberated is proportional to the intensity of light irrespective of the frequency used. This nature of being proportional is in it self an indication of one to one interaction between photon and electrons which have interacted with each other and in turn pointing towards particle nature.
The above inferences drawn form the results made Einstein to propose the Einstein’s photoelectric theory borrowing the concept of Planck’s radiation theory, which mathematically can be presented as below.
hν = W+ (½)mv max2
Where ν is the frequency associated with the incident light, h being the Planck’s constant and hence hν is the energy associated with the photon. W is the work function given by hν0 with ν0 being threshold frequency. The last term in the above equation is associated kinetic energy with the liberated electrons.The general misunderstanding at lower level and some of the statements made in the literature stating that the energy hν is partly used to liberate an electron and the remaining energy is converted into kinetic energy, which is very much against the statement of quantization of energy. Rather it should be understood that the whole of the energy hν is transferred to the electron through a perfect elastic collision and this electron which gets excited converts this energy into the mechanical energy which is used for the ejection of electrons and its kinetic energy.
a) Results obtained by varying the frequency of the incident light.
As the frequency increased from a minimum value gradually it was found that below a certain frequency not a single electron was liberated and above this frequency the electrons were liberated. This frequency was named as threshold frequency. This threshold frequency is different for different metals. It is also found that increase in the frequency of the incident light increases the energy of the ejected electrons while their number remained the same. This indicates the transfer of light energy to the electrons in one to one corresponding manner leading to think that the energy associated with each light wave as quanta of energy referred earlier as photon. This one to one interaction is obviously similar to the transfer of momentum from particle to particle involving elastic collision. No release of electron below the threshold frequency indicates the quanta of energy associated with photon is not sufficient to liberate the electron.
b) Variation of intensity of the light below the threshold frequency.
The experiment was also conducted by varying the intensity of light below the threshold frequency. Under, no intensity of light it is found that the electrons are liberated showing that they are not collectively been able to create an energy which is sufficient to knock out a single electron from the metal surface. This non collective effect of light waves clearly indicates the energy transfer is not in favor of wave theory but is in favor of particle theory.
c) Variation of intensity of the light above the threshold frequency.
Above the threshold frequency it has been found that the number of electrons liberated is proportional to the intensity of light irrespective of the frequency used. This nature of being proportional is in it self an indication of one to one interaction between photon and electrons which have interacted with each other and in turn pointing towards particle nature.
The above inferences drawn form the results made Einstein to propose the Einstein’s photoelectric theory borrowing the concept of Planck’s radiation theory, which mathematically can be presented as below.
hν = W+ (½)mv max2
Where ν is the frequency associated with the incident light, h being the Planck’s constant and hence hν is the energy associated with the photon. W is the work function given by hν0 with ν0 being threshold frequency. The last term in the above equation is associated kinetic energy with the liberated electrons.The general misunderstanding at lower level and some of the statements made in the literature stating that the energy hν is partly used to liberate an electron and the remaining energy is converted into kinetic energy, which is very much against the statement of quantization of energy. Rather it should be understood that the whole of the energy hν is transferred to the electron through a perfect elastic collision and this electron which gets excited converts this energy into the mechanical energy which is used for the ejection of electrons and its kinetic energy.
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