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# Photoelectric effects

الكلية كلية التربية للعلوم الصرفة     القسم قسم الفيزياء     المرحلة 3
أستاذ المادة فؤاد عطية مجيد       07/11/2016 21:37:59
2.7 Photoelectric effect
The quantum nature of light had its origin in the theory of thermal radiation and was strongly reinforced by the discovery of the photoelectric effect.
In figure 2.1, a glass tube contains two electrodes of the same material, one of which is irradiated by light. The electrodes are connected to a battery and a sensitive current detector measures the current flow between them.
The current flow is a direct measure of the rate of emission of electrons from the irradiated electrode.

For a given electrode material, no photoemission exists at all below a certain frequency of the incident light. When the frequency increases, the emission begins at a certain frequency. The frequency is called threshold frequency ( ) of the material. The threshold frequency has to be measured in the existence of e.m.f. (electromotive force) as at such a case the photoelectrons have no kinetic energy to move from the cathode to anode. Different electrode material has different threshold frequency.

2.The rate of electron emission is directly proportional to the intensity of the incident light.
3.Increasing the intensity of the incident light does not increase the kinetic energy of the photoelectrons.
4.There is no measurable time delay between irradiating the electrode and the emission of photoelectrons, even when the light is of very low intensity. As soon as the electrode is irradiated, photoelectrons are ejected.
5.The photoelectric current is deeply affected by the nature of the electrodes and chemical contamination of their surface.

(1) In 1905, Einstein solved the photoelectric effect problem by applying the Planck’s hypothesis. He pointed out that Planck’s quantization hypothesis applied not only to the emission of radiation by a material object but also to its transmission and its absorption by another material object. The light is not only electromagnetic waves but also a quantum. All the effects of photoelectric emission can be readily explained from the following assumptions: The photoemission of an electron from a cathode occurs when an electron absorbs a photon of the incident light;
(2) The photon energy is calculated by the Planck’s quantum relationship: E = h?.
(3) The minimum energy is required to release an electron from the surface of the cathode. The minimum energy is the characteristic of the cathode material and the nature of its surface. It is called work function ( ).
The equation for the photoelectric emission can be written out by supposing the photon energy is completely absorbed by the electron. After this absorption, the kinetic energy of the electron should have the energy of the photon. If this energy is greater than the work function of the material, the electron should become a photoelectron and jumps out of the material and probably have some kinetic energy.

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