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) through an anode with a hole in it. A second anode in the shape of a Maltese cross attracts electrons. The phosphor-coated area at the end of the bulb glows green as the electrons cause the phosphor to emit light. The electrons travel in straight lines, so the Maltese cross casts a shadow.