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X-rays

Short-wavelength, invisible radiation that passes through soft tissue but is stopped far more by bone and metal.

X-rays are electromagnetic waves with wavelengths far shorter than visible light, which gives them enough energy to pass through many materials that light cannot. Wilhelm Röntgen stumbled on them in November 1895 while experimenting with electric discharge tubes, when a screen across his laboratory began to glow. He found the rays passed through black paper, a thousand-page book and wood, but that two millimetres of lead stopped them.

An X-ray picture is really a shadow. Different materials absorb the beam to different degrees: dense, calcium-rich bone and metal absorb much more than muscle, fat or skin. Where the rays get through, the photographic plate darkens; where they are blocked, it stays pale. On 22 December 1895 Röntgen made such an image of his wife's hand, showing her bones and her wedding ring. In 1901 he received the first Nobel Prize in Physics.

A CT scan (computed tomography) takes many X-ray images from different angles and uses a computer to rebuild a three-dimensional picture, slice by slice. The same approach lets researchers look inside objects that cannot be opened, such as fossils or fragile, rolled-up scrolls.

X-rays are ionising radiation. A single ordinary X-ray gives only a small dose, but unnecessary exposure is avoided.

See it in action

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  • "I Have Seen My Death": The First X-Ray of a Person 1895 · Würzburg, Germany
  • The 5,300-Year-Old Murder Found on an X-Ray 1991 found, July 2001 solved · Ötztal Alps, Italy/Austria border
  • The CT Scan Was Born Part-Time in a Cape Town Hospital 1955–1979 · Groote Schuur Hospital and University of Cape Town
  • The DNA Test That First Freed an Innocent Man 10 Sept 1984 discovery; 1986–1988 case · Leicester and Leicestershire, England
  • The Doctor Who Pushed a Tube Into His Own Heart 1929 · Eberswalde, Germany
  • The Scan That Cleared Tutankhamun's 'Murderer' January 2005 · Valley of the Kings, Luxor, Egypt
  • The Star Cairo Saw Three Times the Size of Venus 1006 · Cairo, Fatimid Egypt

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