Cathode rays are invisible radiation emitted (given off) by the cathode (negative electrode) of a Crookes tube (see Crookes tube). The rays can be created by connecting the cathode and the tube's anode (positive electrode) to a high-voltage source of electric energy. Part of the glass of the tube is coated, usually with zinc sulfide. Where cathode rays strike the coating, the coating emits yellowish-green light. When a metal plate is placed in the path of the rays, a boldly defined shadow is created in the light at the end of the tube. Cathode-ray tubes are used in television sets, computer monitors, radar displays, and scientific instruments.