Cathode Ray Tube

Cathode Ray Tube:

The basis of pictures in a TV or waveform measurements on an oscilloscope is the cathode ray tube: a beam of electrons, deflected by vertical and horizontal electric fields, impinging on a phosphorescent screen.

When the electrons strike the screen, they excite the molecules in the screen to higher energy levels. As the screen molecules lapse back to their ground state, photons are emitted, giving dots of light on the screen.

All together, the dots of light represent the voltages guiding the electrons. If color is required, beams of different energies are used to excite the atoms in the screen to different energy levels, therefore giving different photons (of different energies and therefore different colors) as the molecules decay.