PHOTOELECTRIC EFFECT_PART 01

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PHOTOELECTRIC EFFECT: In 1887, Heinrich Hertz discovered photoelectric effect while studying emission of electromagnetic waves by spark discharge. He observed that if cathode is irradiated with ultraviolet radiations, spark of high voltage passes through the gap between the electrodes. In 1888, Hallwachs further developed this idea. He observed that when negatively charged zinc plate with an electroscope is irradiated with ultraviolet light, negative charge on the plate decreased. If neutral plate was used, it became positive and a positive plate more positive. He concluded that under the effect of ultraviolet light, negatively charged particles are emitted by the plate and called them photo electrons. When light of sufficiently high frequency is made incident on photosensitive surface, it emits photo electrons which are accelerated towards the a positive potential plate and current of an order of micro ampere flows through the circuit. For emission of photo electrons, the frequency of the incident light should be more than certain minimum value called threshold frequency ϑo, whose value depends on the type of the metal. For most of the metals, threshold frequency lies in ultraviolet band of electromagnetic spectrum. The alkali metals are most photosensitive and produce photoelectric effect even with ordinary visible light.The last crucial discovery Before Einstein's explanation was made in 1902 by Philip Lenard. He found that electrons are emitted from the metals with range of velocities and maximum kinetic energy Kmax of photoelectrons did not depend on the intensity of exciting light. Kmax increases with the increase of frequency

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