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As you have already studied that when current flows through a LED then it emits light. A photo sensitive diode functions just opposite of the LED. In it, current flows on falling of light, when there is no light on the photo sensitive diode then there is no flow current through it. As soon as light falls on it, it starts conducting. Current through the photo sensitive diode increase with the light falling on it. Just like a ordinary diode, photo sensitive diode also do not conducts during reverse bias but on getting light it conducts on reverse bias and works like a simple diode on forward bias. In this way it can be said that photo sensitive diode is a reverse biased diode. There is a window of glass or plastic on it through which light reaches the diode. In the fig. 35 graphical symbol of diode is shown. An .arrow indicates the direction of light. Generally, reverse supply is given to it which using it. This reverse supply must not be greater than the capacity of the diode. If it raises from the 1/10 of its capacity than very small but constant current flows through the photo diode. This is known as DARK CURRENT. On its reverse voltage capacity this diode works as open circuit. Due to this reverse bias, the depletion layer in the diode larger when light falls on the diode than electron hole pair are produced, as a result increase reverse current starts flowing. This reveres current is known as LIGHT CURRENT. Photo diodes have many functions. In many types of alarms like burglar alarm, fire alarm etc it is used. Besides this it is also used as a light operated switch.