Write and explain methods to obtain emission of electron from metal.

Explanation

From metal surface emission of electron can be obtained by following methods.
(1)Thermionic emission:When metals are heated (current is passed through filament of metal)electrons are given sufficient heat (theramal) energy,electrons escape from the metal such emission is called thermionis emission.
Example:From diode triode or TV tube electrons are emitted by using thermionis emission.
(2)Field emission:When metal surface are applied electric field of `10^(8)(V)/(m)`,electron field emission.
(3)Photoelectric emission:When on a clear metal surface incident radiation of proper frequency,electron are emitted from it.Such emission is called photoelectric emission.
Electrons emitted are called photoelectron.
Additional information:
(4)Secondary emission:When high speed electron collide on metal surface it will impact some energy to free electron in meta.If energy of electron exceed work function of metal,electron will escape from metal.Electron emitted by such method are called secondary electron and such emission is called secondary emission.