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 o