Cathode ray tube. Illustration showing heating of a cathode (red filament) to emit electrons in a cathode ray tube. These electrons are focussed into a beam (or cathode ray, blue) through an anode with a hole in it. The cathode ray is seen by the ionisation of the gas around it. The phosphor-coated screen (round, at right) glows as the electrons cause the phosphor to emit light. Applying magnetic fields across the x and y plates deflects the electron beam.