In 1895, German scientist W C Roentgen began studying the phenomena accompanying the passage of an electric current through a gas of extremely low pressure. His work on cathode rays (the electric current established in highly rarefied gases using very high tension electricity) led him to the discovery of a new and different kind of ray.
On the evening of November 8, 1895, Roentgen was working with a cathode ray tube, similar to fluorescent light bulbs, in his laboratory. He evacuated the tube of all air, filled it with a special gas (Barium Platinocyanide), and passed a high voltage electric gas through it. When he did this, the tube produced a fluorescent glow. Roentgen shielded the tube with heavy black paper, and found that a fluorescent green light could be seen coming from a screen set a few feet away from the tube.
He realised that he had produced a previously unknown ‘invisible’ light or ray, that was being emitted from the tube and was capable of passing through the heavy paper covering the tube.
During subsequent experiments he found that objects of different thicknesses interposed in the path of the rays showed variable transparency to them when recorded on a photographic plate. Roentgen discovered that the x-ray would pass through the tissue of humans leaving the bones visible. When he immobilised for some moments the hand of his wife in the path of the rays over a photographic plate, he observed an image of his wife’s hand which showed the shadows thrown by the bones of her hand and that of the ring she was wearing, surrounded by the penumbra of the flesh, which was more permeable to the rays and therefore threw a fainter shadow. This was the first ‘Roentgenogram’ ever taken. Through additional experiments, he showed that the new rays were produced by the impact of cathode rays on a material object. He named the new ray X-ray. In Mathematics ‘X’ is used to denote the unknown quantity. And hence the name X-ray.
The news of Roentgen’s discovery spread quickly throughout the world. Scientists everywhere duplicated his experiment because the Cathode ray tube was very well known during that time.
In early 1896, X-rays were being utilised clinically for capturing bone fractures and gunshot wounds.
Later scientists showed that X–rays had the same electromagnetic nature as light but differed from it only in the higher frequency of their vibration.