Electron emission from cathode of electric light source
We know that all objects contain a large number of electrons, but the energy of these electrons under normal conditions is not enough to escape from the object. Therefore, to release them from the object, they must be given additional energy to overcome the force that hinders their escape represented by the work function. There are many ways to make the electrons inside the object obtain enough energy to escape from the surface, mainly heating, positive ion bombardment, electron impact and application of strong electric field; the corresponding electron emission is called thermal electron emission, secondary electron emission, electron impact emission and field emission. In electric light sources, according to the form of electron emission, the most commonly used is the thermal electron emission cathode, and there are a few secondary electron emission cathodes. Therefore, we only discuss these two cathodes.