Photoelectric Cell

Photocell (Photoelectric cell)
A device which converts light energy into electrical energy is called photo- electric cell or simply photo cell. It is also known as electric eye.
ü  Photoelectric cell works on the principle of photoelectric effect.
It consists of an evacuated glass or quartz bulb containing cathode C (emitter) and a wire loop anode A (collector). The cathode is a semi-cylindrical photosensitive metal plate C on which a layer of photo- sensitive alkali metal is deposited.
                The external circuit connected to anode and cathode consists of a high tension battery and a micro ammeter () as shown in figure.
                When light is incident on the cathode, it emits photoelectrons which are attracted by the anode. The photoelectrons constitute a small current which flows through the external circuit.




Note: (a) The photoelectric current can be increased by filling some inert gas like Argon into the bulb. The photo-electrons emitted by cathode ionize the gas by collision and hence the current is increased. The positive ions move towards the cathode while negative ions and electrons move towards the anode.
(b) In the absence of light there will be no conduction in the photo tube.

Applications:

(1)Automatic fire alarm: When fire breaks out, light falls on the photo cell and causes the photo emission. Photoelectrons cause the flow of current and a bell connected to it starts ringing.

(2) Automatic burglar alarm: when a burglar (thief) crosses the beam of U.V. light, the emission of photoelectrons stops and bell start ringing.

(3) Automatic door opener: A person approaching a doorway interrupt light beam which is incident on a photocell. The change in photocurrent is used to start a motor which opens the door.


(4) To measure the intensity of light: Light meters in Photographic cameras make use of photocells to measure the intensity of light.
                      
                 
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