CBSE NOTES CLASS 12 PHYSICS
CHAPTER 11 DUAL NATURE OF RADIATION AND MATTER
Cathode rays are the stream of fast moving electrons.
The minimum energy required for the electron emission from the metal surface can be supplied to the free electrons by
(i) Thermionic emission: By suitably heating, sufficient thermal energy can be imparted to the free electrons to enable them to come out of the metal.
(ii) Field emission: By applying a very strong electric field (of the order of 108 Vm–1) to a metal, electrons can be pulled out of the metal, as in a spark plug.
(iii) Photo-electric 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.