PHOTOELECTRIC EFFECT PART 02

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EXPERIMENTAL STUDY OF THE PHOTOELECTRIC EFFECT:The experimental arrangement to study the photoelectric effect is shown in the figure. It consists of two electrodes in an evacuated tube. The ultraviolet rays entering quartz window are incident on the photo-sensitive surface C, known as cathode. Collector A is kept at different positive or negative voltage with respect to C with the help of a potential divider and P.D. between the electrodes is measured by a voltmeter V. The photocurrent can be measured with the help of micrometer. Quartz window is used as glass is opaque to UV light. The photoelectric effect can be studied with reference to the frequency and intensity of the incident light, number of photoelectrons emitted and their maximum energy. When proper positive potential is applied to collector C, all the photoelectrons are attracted towards it and the maximum current recorded by the micro-ammeter gives an idea of the number of photo electrons.
Effect of intensity of light on photocurrent: Effect of intensity of light on photocurrentWith collector plate at fixed positive potential and keeping the frequency of the incident radiation constant, the intensity of incident light is varied and corresponding photocurrent is noted. The variation of photocurrent with intensity of incident radiation is as shown in the graph. It is a straight line indicating linear variation and implies that number of photoelectrons emitted per second is directly proportional to the intensity of incident light.
Effect of potential on photocurrent: When negative potential is applied to the collector, only such electrons which have sufficient energy to overcome the negative potential may reach the collector. On making collector more negative, photoelectric current decreases and becomes zero at or lower than certain negative potential. This minimum negative potential of the collector with respect to the photosensitive surface at which photoelectric current becomes zero is called stopping potential. It is denoted by Vo.According to the definition of stopping potential, electron on the surface of the photosensitive surface having maximum velocity, v max just reaches the collector plate overcoming stopping potential V0. In the process, the work eVnot done by it is at the cost of its kinetic energy.The graph of photoelectric current versus the potential of collector with respect to the emitter for different intensities of incident light for a given frequency is as shown in the figure. The graph implies that the saturation photo current increases with intensity of light while stopping potential remains same. Thus stopping potential is independent of intensity of incident light.