Photoelectric cell. A device which can convert light energy into electrical energy known is photoelectric cell. Photoemissive cell is a photoelectric cell.
Photoemissive cell : It consists of two electrodes, a cathode C and an anode A enclosed in a highly evacuated glass bulb. The cathode C is a semi-cylindrical plate coated with a photosensitive material, such as a layer of cesium deposited on silver oxide. The anode A is in the form of a wire, so that it does not obstruct the path of the light falling on the cathode.
When light of frequency above the threshold frequency for the cathode surface is incident on the cathode, photoelectrons are emitted. If a potential difference of about 10 v is applied between the anode and cathode, the photoelectrons are attracted towards the anode connected in the circuit will record the current.
The chief advantages of this type of photocell are that there is no time, lag between the incident light and the emission of photoelectrons and that the photoelectric current is proportional to the intensity of light. The cell is extremely accurate in response. Hence, it is used in television and photometry.
The current may be increased by a factor of about 5 by filling the tube with an inert gas at a pressure of a few mm of mercury. When the potential difference between the electrodes exceeds the ionisation potential of the gas, the emitted photoelectrons ionise the gas atoms and now a larger current flows.
Practical Use. These cells are quite good for are in a film projector because it can follow the change in light intensity from sound track.
