Photoelectric Cell

 

Photoelectric Cell

Photoelectric cell is an electronic device which converts light energy into electrical energy. It is also called an electric eye or photo tube.

Principle

Photoelectric cell works on the principle of photoelectric effect.

Construction

Photoelectric cell consists of an evacuated glass tube in which there are two electrodes, emitter(C) and collector(A). The emitter is a semi-cylindrical photosensitive metal plate(works as cathode) and the collector is a wire loop(works as anode). The glass tube is fitted on an insulating base and metallic pins are provided at the base for external connection. Through metallic pins, the emitter is connected to the negative terminal and the collector is connected to the positive terminal of a battery (B) provided in the external circuit.

Working

When light of suitable frequency (greater than threshold frequency ν₀) is made to fall on the cathode, photo electrons are emitted from it, which are attracted by anode because it is connected to the positive terminal of the battery B. So a current starts flowing in the circuit which is shown by the deflection in the micro ammeter(μA). As soon as the light falling on the cathode is cut-off, no photoelectrons are ejected from the cathode and hence there is no current in the external circuit.

 

Applications

1. In sound reproduction :- Photoelectric cells are used for the reproduction of sound recorded on movie film.
2. Photoelectric cells are used in counting devices to count the persons entering in a hall, provided the come one by one.
3. These are used in burglar alarms and also in fire alarms.
4. In street light circuits, photo-cells are used to switch on and off the lights automatically at dusk and dawn.
5. They are used in photometry to compare the illuminating powers of two sources of light.
6. They are also used in public buildings for automatically opening the doors.
7. Photoelectric cells are used for the determination of Planck’s Constant.
8. These are also used in television cameras for telecasting scenes and in photo telegraphy (transmission of pictures, print, etc., by means of radio or telegraphy).

 

 

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