Cathode rays are flows of electrons in vacuum tubes, i.e., glass tubes equipped with at least two electrodes, a cathode (negative electrode) and an anode (positive electrode). When the cathode is heated, it emits electrons that travel to the anode. If the inner glass walls behind the anode are covered with a fluorescent material, they glow brightly.

The cathode rays travel towards the anode in a straight line and continue beyond it for a certain distance. This phenomenon was studied by eminent physicists at the end of the 19th century, with a Nobel Prize in Physics awarded in 1905 to Philipp Lenard. Special tubes were developed for the study of these rays by William Crookes, called Crookes tubes in his honor. Soon it was clear that the cathode rays were formed by real bearers of electricity, since then known as electrons.

The cathode rays propagate in a straight line in the absence of external influences and regardless of where the cathode is located, but are diverted by electric or magnetic fields. The refinement of this idea is the cathode ray tube, which was key in televisions, oscilloscopes, and vidicon television cameras, before the emergence of current digital technologies with the use, among other things, of flat plasma and LED screens.

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