When people talk about rays, we usually think about electromagnetic waves. Think of lesser beams, beams of light, X-rays, or gamma radiations. In physics there are “rays” that have nothing to do with the light or electromagnetism. A simple example is radioactivity. This often takes the form of gamma radiation, but there are also alpha and beta rays which are respectively a stream of helium and a stream of electrons. The same applies to the cosmic rays, which were discovered just over a century ago.