By Sunil Bhardwaj

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Photo cells are the devices which converts the radiant light in to electric current.

Photo Emissive Cells: The simplest form of it consists of an evacuated glass tube fit with photo cathode. The photo cathode is concave shaped. It is made of Cesium (Cs) or it is made of metal and surface is coat with Cesium or in some cases the surface is coated with Potassium oxide and silver oxide this cathode is called as Photo-cathode. At the centre, there is metallic point. This acts as anode known as photo anode, connected in series with battery, galvanometer, resistance, and amplifier.

TLC

When the transmitted light from sample solution is incident on photo cathode, as the surface is alkali metal, then quickly electrons are ejected out and these electrons are attracted towards anode. This flow of electrons will produce current. If the current value is small then it is to be amplified by amplifier. The current produced is directly proportional to the number of electrons ejected from cathode and number of electrons ejected is directly proportional to transmitted light incident on it. It means current produced is directly proportional to transmitted light incident on photo cathode.

Photomultiplier Tube (PMT): It is also known as electron multiplier tube. It consists of cylindrical tube which is evacuated. The tube is having one cathode called photo cathode and number of the electrodes called Dynodes which are arranged as shown in the diagram.

TLC

A dynode is electrode with the coating of Cs and Sb or Cs and Silver Oxide. All the dynodes are kept at increasing value of potential. When radiations are incident on photo cathode, it ejects the electrons. These electrons are incident on dynodes. The primary electron gets accelerated in electrostatic field between the cathode and first dynode and then impregnate on the dynode.

As many as TEN secondary electrons may be emitted by dynode for each electron which strike on it. Thus, process is repeated at the several dynodes to which increasing potential is applied.

As a result, the electronic current gets amplified and the pulse of current gets increased by million time greater than the initial current emitted by cathode (Photo Cathode).

Photovoltaic cell or Barrier Layer cells: These cells generate their own e.m.f. and require no external source for operation. The photo voltaic cell consists of metal plate usually made of iron on which a thin layer of selenium is deposited. A thin transparent film of silver is spread over the selenium. It acts as the collector electrode.

TLC

The radiant energy which is incident on the semi-conductor surface i.e. Selenium-Silver surface release the electrons at the silver selenium interface and these electrons pass to the collector electrode. If the galvanometer is connected in the circuit it shows the deflection indicating the current is produced.

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