A cathode ray tube creates a picture by firing a stream of electrons at a phosphorescent screen.
In a cathode ray tube, the "cathode" is a heated filament in a vacuum inside a glass "tube." The "ray" is a stream of electrons that pours off the filament.
This beam of electrons flies through the tube and hits the many small dots of a phosphor — a substance that glows when hit by electrons — that are coated on a flat screen at the other end.
The stream of electrons excites the electrons in the phosphor to higher energy levels. When the electrons drop back to a lower level, they emit the excess energy in the form of light.
By varying the voltage to the anodes and using steering coils, we can control the brightness of the phosphor and direct the beam to various places on the screen. This is what creates the picture.
Black and white cathode ray tubes contain a single phosphor.
Colour cathode ray tubes use red, blue, and green phosphors and three separate electron beams — one for each of the three colours. When each phosphor is struck by its respective electron beam, it glows, creating a color picture.