Cathode Rays, in physics, a stream of electrons given off by the negative electrode, or cathode, of a vacuum tube. A cathode-ray tube (CRT) consists of an electron gun for emitting the electrons; deflecting plates for focusing the rays; and a screen, coated with a material such as zinc sulfide that will glow brightly when struck by the cathode rays. The picture tube in a television set is a form of cathode-ray tube. Other types are used in radar, in electronic measuring instruments, and as graphic-display terminals for computers and word-processing machines.

The rays leave the cathode at a very high speed, moving in a straight line across the tube. They can be bent from their straight path by a magnetic or electrostatic field.