The minimum energy required for the electron emission from the metal
surface can be supplied to the free electrons by any one of the following
physical processes:
(i)
Thermionic emission: By suitably heating, sufficient thermal energy
can be imparted to the free electrons to enable them to come out of the
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(ii) Field emission: By applying a very strong electric field (of the order of
108 V m–1) to a metal, electrons can be pulled out of the metal, as in a
spark plug (iii) Photoelectric emission: When light of suitable frequency illuminates
a metal surface, electrons are emitted from the metal surface These
photo(light)-generated electrons are called photoelectrons