Cathode rays are a phenomenon that is observed in vacuum tubes, i.e. evacuated glass tubes which are equipped with at least two electrodes, a cathode (negative electrode) and an anode (positive electrode), a configuration known as a diode.

When the cathode is heated, it emits some radiation which proceeds to the anode. This can be seen if one coats the inner glass walls behind the anode with a phosphorecent material: One observes that it lights up. Putting a metal shape between the electrodes shows this shape as a shadow on the glowing coating. This means that the cause of the light emission is that the coating is hit by rays which are emitted by the cathode, proceed towards the anode in straight lines, and continue at least partly past it.

This phenomenon has been studied in great detail in the field of physics toward the end of the 19th century, yielding a Nobel prize for Philipp von Lenard.

It was soon understood that the cathode rays consist of the actual carriers of electricity which are now known as electrons. The fact that the rays are emitted by the cathode, i.e. the negative electrode, showed that electrons have negative charge.

Cathode rays (or electron rays as we now also call them) propagate in a straight line in the absence of external influences, but are deflected by electric or magnetic fields (which can be produced by placing high voltage electrodes or magnets outside the vacuum tube, thus the effect of magnets on a TV screen). The refinement of this idea is the cathode ray tube (CRT), also known as Braun's tube (because it was invented 1897 by Ferdinand Braun). The CRT is key to television sets (though alternative display technologies are making inroads), oscilloscopes, and vidicon television cameras.