Wilhelm Röntgen’s 1895 cathode-ray experiment revealed X-rays, which could produce images of bones.
Röntgen was investigating cathode rays in a discharge tube covered to block visible light. He noticed that a nearby fluorescent screen glowed even when the tube was shielded. He concluded that an unknown type of penetrating radiation was being produced and called it X-radiation, using “X” for something unknown.
The radiation passed through soft tissue more readily than through dense materials such as bone. Röntgen soon made an image of his wife Anna Bertha’s hand, showing her bones and a ring. This became one of the earliest famous medical radiographs.
X-rays are electromagnetic radiation with much shorter wavelengths than visible light. Röntgen’s discovery did not involve ultrasound or infrared light. He received the first Nobel Prize in Physics in 1901, and the new technology quickly transformed medical diagnosis.