Page "Cathode" Paragraph 17

In a vacuum tube or electronic vacuum system, the cathode emits free electrons.

Electrons are extracted from metal electrodes either by heating the electrode, causing thermionic emission, or by applying a strong electric field and causing field electron emission.

Electrons can also be emitted from the electrodes of certain metals when light of frequency greater than the threshold frequency falls on it.

This effect is called photoelectric emission.