Photoelectric cells and their applications
The photoelectric cell is a device which converts light energy into electrical energy. The photo electric cells are of three types:
1. Photo emissive cell
2. Photo voltaic cell and
3. Photo conductive cell
In this section, a simple photo emissive cell is discussed. It consists of a highly evacuated bulb B made of glass or quartz. A semi cylindrical metal plate C connected to the negative terminal of a battery, acts as cathode. This plate is coated with a low work function material such as caesium oxide, in order to get large number of photo electrons. A thin platinum wire A is connected to the positive terminal of the battery and kept along the axis of the metal plate C and this serves as the anode as shown in Fig.
When a light of suitable wave length falls on the cathode, photo electrons are emitted, which are attracted by the anode A. The resulting current is measured by a micro ammeter. The current produced by this type of cell is proportional to the intensity of the incident light for a given frequency.
Applications of photo electric cells
1. Photoelectric cells are used for reproducing sound in cinematography.
2. They are used for controlling the temperature of furnaces.
3. Photoelectric cells are used for automatic switching on and off the street lights.
4. Photoelectric cells are used in the study of temperature and spectra of stars.
5. Photoelectric cells are also used in obtaining electrical energy from sunlight during space travel.
6. These cells are used in instruments measuring light illumination.
7. These cells are used in opening and closing of door automatically.
8. Photoelectric cells are used in burglar alarm and fire alarm. In burglar alarm, ultraviolet light is continuously made to fall on the photo-cell installed at the door-way. A person entering the door interrupts the beam falling on the photo-cell. The abrupt change in photocurrent is used to start an electric bell ringing. In fire alarm, a number of photo-cells are installed at suitable places in a building. In the event of breaking out of fire, light radiations fall upon the photo-cell. This completes the electric circuit through an electric bell or a siren which starts operating as a warning signal.