Roentgen discovered X-rays. Wilhelm Conrad Roentgen discovered X-rays while examining cathode rays. He noticed the glowing on an aluminium sheet that was exposed to it. This was even the case when material was placed in between the source and the sheet. He concluded that something invisible must have caused that fluorescence and observed that it also affected photographic films. Those invisible rays he named X-rays. He then quickly realizes that the different capacity of X-ray to penetrate materials and tissues, muscle or bones, caused the appearance of his skeleton hand on the fluorescent screen.