Cathode Ray Tube
Focusing And Deflection Systems
Under normal circumstances, an electron beam produced by the electron gun described above would have a tendency to spread out to form a cone-shaped beam. However, the beam that strikes the display screen must be pencil-thin and clearly defined. In order to form the electron beam into the correct shape, an electrical or magnetic lens, similar to an optical lens, can be created adjacent to the accelerating electrode. The lens consists of some combination of electrical or magnetic fields that shapes the flow of electrons that pass through it, just as a glass lens shapes the light rays that pass through it.
The electron beam in a cathode ray tube also has to be moved about so that it can strike any part of the display screen. In general, two kinds of systems are available for controlling the path of the electron beam, an electrostatic system and a magnetic system. In the first case, negatively charged electrons are deflected by similar or opposite electrical charges and in the second case, they are deflected by magnetic fields.
In either case, two deflection systems are needed, one to move the electron beam in a horizontal direction, and the other to move it in a vertical direction. In a standard television tube, the electron beam completely scans the display screen about 25 times every second.