When the electrons of metals acquire an energy equal to or greater than its work function " Ï• " it leaves the metal. The electron can acquire this energy in the following four ways.
1) Thermionic Emission:
If a metals heated, the electrons acquire an energy greater than "Ï•" and come out of the metal just like the vapours of a liquid which is heated up. This type of the emission is called thermionic emissions.
2) Photo-Electric Emission:
If light of suitable frequency is made to fall upon the metallic surface. Light photons impart some of their energies to the free electrons of the metals which come out of the metal. This phenomenon in which light photons of suitable frequencies knock out electrons of the metals by elastic collisions is called photo-electric emission.
3) Field Emission:
Emissions of electrons can be possible if an intense electric field is applied on the metal surface in a correct direction. Such an emission is known as field emission.
4) Secondary Emission:
If a metal surface is bombarded by electron, which have acquired very high energy, the electrons of the metal receive energy from these primary electrons on elastic collisions and in this way leave the metal surfaces. these electrons which are emitted from the metal surfaces are called secondary electrons and this phenomenon is called secondary emission.
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