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CBSE Class 12th Physics Notes: Dual Nature of Radiation and Matter (Part ‒ I)

Jan 12, 2017 17:00 IST

    CBSE chapter wise notes based on chapter 11, Dual Nature of Radiation and Matter of NCERT textbook are available in this article.

    In this article, students will get important key notes on the topics given below

    Electron Emission

    Methods of Electron Emission

     • Thermionic emission

     • Field emission

     • Photo-electric emission

    Photoelectric Effect

    Hertz’s Observation

    Hallwachs’ and Lenard’s Observations

    Experimental Study of Photoelectric Effect

    Effect of intensity of light on photocurrent

    Effect of potential on photoelectric current

    Effect of frequency of incident radiation on stopping potential

    Laws of Photoelectric Effect

    The complete notes are as follows

    Electron Emission

    Free electron inside the metal surface by the attractive forces of the ions, the electron can come out of the metal surface only if it has got sufficient energy to overcome the attractive pull. A certain minimum amount of energy is required to be given to an electron to pull it out from the surface of the metal. This minimum energy required by an electron to escape from the metal surface is called the work function of the metal.

    Methods of Electron Emission

    Thermionic emission

    By supplying heat, sufficient thermal energy can be imparted to the free electrons which enable them to come out of the metal.

    Field emission

    By applying a very strong electric field (of the order of 108 V m–1) to a metal, electrons can be pulled out of the metal, as in a spark plug.

    Photo-electric emission

    When light of suitable frequency falls on a metal surface, electrons are emitted. These photo (light) generated electrons are called photoelectrons.

    Photoelectric Effect

    Metallic surfaces when illuminated by ultraviolet radiation then electrons are emitted from them. This effect is known as the photoelectric effect. By absorbing energy from the incident electromagnetic radiation, the electrons in the metal escape the attraction of ions in the metal.

    Hertz’s Observation:

    In 1887 by Heinrich Hertz observed that when light falls on a metal surface, some electrons near the surface absorb enough energy from the incident radiation to overcome the attraction of the positive ions in the material of the surface. After gaining sufficient energy from the incident light, the electrons escape from the surface of the metal into the surrounding space.

    Hallwachs’ and Lenard’s Observations

    By a series of experiments, Hallwachs’ and Lenard’s observed that there is a certain minimum frequency, known as threshold frequency, below which no electrons were emitted.

    Experimental Study of Photoelectric Effect

    Schematic diagram to study of phenomenon of Photoelectric Effect is given above. The main observations of the experiments are given below

    Experimental Study of Photoelectric Effect

    Effect of intensity of light on photocurrent

    The photocurrent is directly proportional to the number of photoelectrons emitted per second. This implies that the number of photoelectrons emitted per second is directly proportional to the intensity of incident radiation.

    Effect of intensity of light on photocurrent

    Effect of potential on photoelectric current

    For a given frequency of the incident radiation, the stopping potential is independent of its intensity. In other words, the maximum kinetic energy of photoelectrons depends on the light source and the emitter plate material, but is independent of intensity of incident radiation.

    Effect of potential on photoelectric current

    In the above figure, for a particular frequency of incident radiation, the minimum negative (retarding) potential V given to the plate A for which the photocurrent stops or becomes zero is called the cut-off or stopping potential.

    Effect of frequency of incident radiation on stopping potential

    This implies that greater the frequency of incident light, greater is the maximum kinetic energy of the photoelectrons. Consequently, we need greater retarding potential to stop them completely

    Effect of frequency of incident radiation on stopping potential

    Variation of photoelectric current with collector plate potential for different frequencies of incident radiation is shown in the figure given above.

    Variation of stopping potential with frequency

    Variation of stopping potential V with frequency ν of incident radiation for a given photosensitive material is given above.

    Laws of Photoelectric Effect

    (i) For a given photosensitive material and frequency of incident radiation (above the threshold frequency), the photoelectric current is directly proportional to the intensity of incident light.

    (ii) For a given photosensitive material and frequency of incident radiation, saturation current is found to be proportional to the intensity of incident radiation whereas the stopping potential is independent of its intensity.

    (iii) For a given photosensitive material, there exists a certain minimum cut-off frequency of the incident radiation, called the threshold frequency, below which no emission of photoelectrons takes place, no matter how intense the incident light is. Above the threshold frequency, the stopping potential or equivalently the maximum kinetic energy of the emitted photoelectrons increases linearly with the frequency of the incident radiation, but is independent of its intensity.

    (iv) The photoelectric emission is an instantaneous process without any apparent time lag (∼10– 9s or less), even when the incident radiation is made exceedingly dim.

    CBSE Class 12th Physics Notes: Dual Nature of Radiation and Matter Part - II

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