Photoelectric Effect
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| Discuss Photoelectric Effect PHOTOELECTRIC EFFECT
What is photoelectric effect?
Photoelectric effect is the liberalisation of electrons from a metal surface exposed to any sort of electromagnetic radiation. This effect is a quantum process. Here radiation is regarded as a stream of photons each having an energy hv where h is the plank’s constant and v the frequency of the radiation.
This phenomenon of photoelectric effect is studied by using the following experimental setup.
In this experiment, monochromatic light of a single frequency is focused on the anode of an evacuated glass tube. The anode is made up of the metal whose behavior under exposure to light is being studied. The flow of electrons in the external circuit indicates the flow of electrons emitted from the anode surface inside the tube. This is possible if the electrons are emitted with energy large enough to overcome the retarding potential between the anode and the cathode.
In this case the free electrons in the metallic anode can absorb energy from the electromagnetic waves impinging upon them. After sufficient energy has been absorbed, free electrons inside the metal should be able to overcome the combined potential barrier offered by the metal surface and the retarding potential across the tube. When the current in the external circuit is measured by varying the intensity, frequency and the retarding potential between the anode and the cathode, certain effects are observed which cannot be reconciled with the classical wave theory of light and its absorption by electrons.
The following observations were made
1. The maximum kinetic energy of the electrons with which they leave the anode can be measured by adjusting the retarding potential till the current in the external circuit is reduced to 0. Thus if V be the Cut-Off voltage then eV is the kinetic energy of the electrons.
2. The energy distribution of the emitted electrons is independent of the intensity of the absorbed light. Thus more photoelectrons are emitted for higher intensities of light, although the maximum kinetic energy with which the electrons are emitted remains unchanged. In fact with light of very low frequency some electrons with the same kinetic energy are emitted.
3. It is observed that within experimental limits that there is no time lag between the arrival of light at the anode and the emission of electron from the metal surface, although experimentally it has been calculated that the time lag is less than 109 s.
4. For a given metal the emission of electrons does not take place if the frequency of the light is less than a certain value. this frequency is referred to as the threshold frequency for that metal.
NOTE: Apart from the liberation of electrons from the metal surface other phenomenon are also referred to as photoelectric effect. These are photoconductive and photovoltaic effects.
