What phenomenon occurs when electrons are emitted from the cathode in a CRT?

The phenomenon that occurs when electrons are emitted from the cathode in a cathode ray tube (CRT) is thermionic emission. This process involves the heating of the cathode, which causes it to release electrons due to the thermal energy overcoming the binding forces that hold them in the material. At elevated temperatures, the electrons gain enough energy to break free from the atomic structure and flow toward the anode, creating a stream of electrons known as a cathode ray.

Thermionic emission is fundamental to the operation of CRTs, as the emitted electrons are then accelerated and directed to create images on the screen. This emission is different from other processes of electron emission; for example, photoemission involves the release of electrons due to the absorption of light, and field emission occurs when a strong electric field allows electrons to tunnel out of a material. Charge emission is not a standard term used in this context, which helps further clarify why thermionic emission is the appropriate answer. Understanding these distinctions is critical for grasping how CRT technology operates.