Write and explain the methods to obtain the emission of electrons from a metal surface.

(N/A) The emission of electrons from a metal surface can be obtained by the following methods:
$(1)$ Thermionic emission: When metals are heated (e.g.,by passing a current through a metal filament),electrons are provided with sufficient thermal energy to overcome the work function and escape from the metal surface. This process is called thermionic emission.
Example: Electrons are emitted in diodes,triodes,or $TV$ tubes using thermionic emission.
$(2)$ Field emission: When a very strong electric field of the order of $10^{8} \text{ V/m}$ is applied to a metal surface,electrons are pulled out from the surface. This process is called field emission.
$(3)$ Photoelectric emission: When electromagnetic radiation of a suitable frequency (above the threshold frequency) is incident on a clean metal surface,electrons are emitted from it. This process is called photoelectric emission,and the emitted electrons are known as photoelectrons.