Electron emission is the process by which electrons are released from a material, typically a metal, when energy is applied. This energy can come from various sources, such as heat, light, or electric fields. When the energy is sufficient to overcome the attractive forces holding the electrons in the material, they are emitted into the surrounding space. There are different types of electron emission, including thermionic emission, where heat causes electrons to escape, and photoemission, where light energy is used. Another type is field emission, which occurs in strong electric fields. These processes are essential in various applications, including vacuum tubes and semiconductors.