PHOTOELECTRIC EFFECT

In 1887, 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 and called them photo electrons. The evacuated photo tube containing photo sensitive surface S whichis kept at a negative potential with respect to collector C. When light of sufficiently

high frequency is made incident on S, it emits photo electrons which are accelerated towards the collector and current of an order of micro ampere flows through the circuit. For emission of photoelectrons, the frequency of the incident light should be more than some minimum called threshold frequency (f0) whose value depends on the type of the metal. For most of the metals, threshold frequency lies in ultraviolet band of electromagnetic spectrum. For example, the threshold frequencies of zinc, cadmium, magnesium lie in the ultraviolet region while those of alkali metals like lithium, sodium, potassium, rubidium, calcium lie in the visible region.