A cathode ray is a stream of electrons emitted from the negative electrode, or cathode, in a vacuum tube. When high voltage is applied, these electrons accelerate towards the positive electrode, or anode, creating a visible beam. This phenomenon is fundamental in the study of atomic structure and the development of electronic devices. Cathode rays were first discovered in the late 19th century and played a crucial role in the development of television and cathode ray tubes (CRTs). Scientists like J.J. Thomson used cathode rays to identify the electron, leading to significant advancements in physics and chemistry.