Final answer:
Hans Geisler observed that a gas discharge tube glowed when electricity was passed through it, which led to the discovery of cathode rays—streams of electrons responsible for the emission of light and electromagnetic radiation.
Step-by-step explanation:
When Hans Geisler, a German inventor, passed electricity through a gas discharge tube, he noticed that the gas inside the tube glowed. This phenomenon is caused by the high voltage applied to the electrodes in the tube causing the flow of electricity through the gas, which then emits light. These observations by Geisler and the subsequent investigations by other scientists like William Crookes and J.J. Thomson led to the discovery that the light emission was associated with cathode rays, a stream of electrons being freed from atoms and molecules and accelerated from the cathode to the anode. This emission culminated in what we now understand as the visualization of electromagnetic radiation and was crucial in identifying the presence of electrons and measuring their properties, such as charge and mass-to-charge ratio.