Today in 1895, a physicist named Wilhelm Conrad Rontgen became the first human being to observe X-rays. It was an enormous breakthrough that would radically change multiple fields of science, especially medical science.
The discovery was actually accidental. Rontgen was testing whether cathode rays had the ability to pass through glass when he observed an unexpected glow coming from a chemical-coated screen that was placed nearby in his lab. X-rays were responsible for the luminescence. Rontgen dubbed the unknown radiation the "X-ray" due to its then unknown properties.
An X-ray is an electromagnetic energy wave that behave much like light. Their wavelengths are about a thousand times shorter than light.
Rontgen committed himself to studying the new X-rays. His experiments demonstrated that the X-ray could penetrate flesh, but not bone. This would set the stage for the X-ray's primary use, as a medical diagnostic tool.
X-rays were first used in this capacity in 1897, during the Balkan War. They helped located bullets inside of wounded soldiers, and to identify broken bones. X-rays are still commonly used for these and other similar purposes in medical contexts today.
As with any new technology, X-ray devices were very useful but came freighted with health risks that were not understood at the time. There's a reason you wear a lead apron when you get medical X-rays: they are a form of radiation that can have bad impacts on your health if you're overexposed.
Unfortunately, scientists and doctors believed that X-rays were completely harmless. Within a few years of X-ray technology being used, patients started presenting with radiation burns and skin damage. Eventually, someone died from X-ray exposure.
That person was Clarence Dally, Thomas Edison's assistant. Dally had done a lot of work with X-rays and it eventually claimed his life, in the form of skin cancer that was directly attributable to his radiation exposure. Science still did not fully understand the health risks associated with X-rays, but Dally's death placed doubt in many minds.
From the thirties through the fifties, X-rays were still overused. Shoe stores commonly offered shoe-fitting fluoroscopes that would X-ray customer's feet. The gimmick was effective for selling shoes but was a public health hazard. The practice was finally banned in the fifties.
Rontgen was awarded the first ever Nobel Prize in Physics in 1901 for his discovery. Rontgen never appeared to be motivated by money and the X-ray was deliberately left unpatented.
X-rays are still in wide use. Wide, but cautious, use.