What r the differences between Photo-electric emission, Secondary emission and Field emission?

Hi,

Photoelectric emission:

Emission of free electrons from a metal surface by falling light (or any other electromagnetic wave which has an energy greater than the work function of the metal) is called photoelectric emission. The electrons so emitted are called photoelectrons. This is explained in detail as under. 

Field emission:

When a conductor is put under strong electric field the free electrons on it experience an electric force in the opposite direction of field. Beyond a certain limit electrons start coming out of the metal surface. Emission from a metal surface by this method is called the field emission.

Secondary emission:

Emission of electrons from a metal surface by the bombardment of high speed electrons or other particle is known as secondary emission.

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