At room temperature the free electrons move randomly within the conductor, but they don’t leave the surface of the conductor due to attraction of positive charges. Some external energy is required to emit electrons from a metal surface. Minimum energy is required to emit the electrons which are just on the surface of the conductor. This minimum energy is called the work function( W) . The work function is the property of the metallic surface.

The energy required to liberate an electron from metal surface may arise from various source such as heat, light, electric field etc. Depending on the nature of source of energy, the following methods are possible.

(i) Thermionic emission: The energy to the free electrons can be given by heating the metal, the electrons so emitted are known as thermions.

(ii) Field emission: When a conductor is put under strong electric field the free electrons on it experience an electric force in the opposite direction of field. Beyond a certain limit electrons start coming out of the metal surface. Emission from a metal surface by this method is called the field emission.

(iii) Secondary emission: Emission of electrons from a metal surface by the bombardment of high speed electrons or other particle is known as secondary emission.

(iv) Photoelectric emission: Emission of free electrons from a metal surface by falling light (or any other electromagnetic wave which has an energy greater than the work function of the metal) is called photoelectric emission. The electrons so emitted are called photoelectrons. This is explained in detail as under.

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