Cathode ray Tube Phosphor


In the cathode ray tube, how can light be emitted when electrons hit on the fluroscent screen?


In the CRT the inside face of the tube is coated with material called a phosphor. When an electron strikes a phosphor atom it raises one of that atom's electrons to a higher energy level, leaving the atom in what is called an excited state. After a short time the excited atom returns to its original energy level. The conservation of energy requires that something carry away the energy difference between the excited and ground state of the phosphor atom and that something turns out to be a light photon. The color of the emitted light is determined by the difference in energy between the excited and ground states of the particular phosphor atom.