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CBSE Class 12 Chemistry Dual Nature of Radiation and Matter Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.
*Electron emission refers to the liberation of free electrons from the surface of a substance aided by the external energy transferred to the electrons.
*It tends to occur on metals because metals are rich in free electrons.
*The amount of external energy required to emit an electron from the metal surface is known as work function, which depends on the nature of the metal.
*The work function is usually expressed in electron volt (eV) unit.
Methods for obtaining electron emission from the metal surface
*Thermionic Emission: Filaments get heated while passing current through them and on sufficient heating electrons are emitted. Triodes, cathode ray tubes etc. are based on this principle.
*Field Emission: In this process, free electrons are pulled out by applying strong electric fields of the order of 108 Vm–1.
*Photoelectric Emission: When electromagnetic radiation of sufficiently high frequency falls on a clean metal surface, free electrons are emitted from the surface. The electrons so emitted are called the photoelectrons.
*Hertz observed that if cathode is irradiated with ultraviolet radiations, spark of high voltage passes through the gap between the electrodes.
*Lenard’s experiments lead to the discovery of a current between the electrodes assembled in an evacuated glass
tube, when the emitter plate was illuminated using UV radiations. The current existed only when UV radiations were incident on the same.
*Hallwachs further studied this and found that when negatively charged zinc plate with an electroscope is irradiated with ultraviolet light, negative charge on the plate decreased or vanished. If neutral plate were used, it became positive and a positive plate more positive. This lead to the conclusion that negatively charged particles were emitted from the plate.
*Photoelectric effect refers to the phenomenon of emission of electrons from the surface of metals (like Zn, Cs, K etc.) when irradiated with high frequency electromagnetic radiations.
*For emission of photoelectrons, the frequency of the incident light should be more than certain minimum called threshold frequency ( 0 n ) whose value depends on the type of the metal.
*For most of the metals, threshold frequency lies in ultraviolet band of electromagnetic spectrum. A few metal have the same in the visible region.
The experimental arrangement to study photoelectric effect consists of an evacuated glass tube with cathode C and anode A near either ends. UV rays from the source S fall on C and electrons are emitted. A circuit can be created so that the photoelectrons initiate a current. The potential of electrode A can be adjusted with reference to the electrode C. The following observations have been made.
*The photoelectric current is directly proportional to the intensity of light. When the frequency of incident radiation
and the accelerating potential remain the same, the number of photoelectrons emitted per second is directly
proportional to the intensity of the incident radiation.
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