Electron Emission- Dual Nature of Radiation and matter

Free Electrons move randomly in a conductor due to attraction of positive charges. So some energy is required to emit electrons from metal surface. Minimum energy is required to emit the electrons on the surface of conductor. This minimum energy is called as a work function (W). the work function is the property of the metallic surface.
The energy may from various sources like a heat, light, electric field etc. Depending on the source of energy, categorized into 4 types.
Thermionic emission: the energy of free electrons taken from the heating the metal, electrons so emitted are known as thermions
Field emission: When a conducted placed in a strong electric field the free electrons experiences an electric force in the opposite direction of field. This method of emission is called as field emission.
Secondary emission: Emission of Electrons from a metal surface by the bombardment of high speed electrons or other particle is known as a secondary emission.
Photoelectric emission: Emission of free electrons from a metal surface by falling light is called as photoelectric emission. Emitted electrons are called as photo electrons.