X-ray is an electromagnetic wave with a wavelength of about 10-10 meters, which is beyond the range of wavelength visible to the human eye. It is often used in hospitals and dental clinics to produce images of patient's internal organs, bones or teeth on a photographic plate.
- X-rays were discovered in 1895 by Wilhelm Conrad Röntgen, who received the first Nobel Prize in Physics in 1901. 
Ron Curtis wrote:
- X-rays are part of the electromagnetic spectrum, which includes radio waves, microwaves, and visible light. They have an extremely short wavelength or high frequency, thus giving them high energy. X-rays can penetrate most materials except lead shielding. They are used in medicine and industry to examine structural problems. When they were discovered, the "X" stood for "unknown," because they were so mysterious. The name has been used ever since.
During the 1930s, 1940s and 1950s, many shoe stores used an X-ray machine to demonstrate how well a shoe fit the feet of the customer. By 1960 the practice had been discontinued in the United States because of safety concerns. The machines were still in use in Canada and Great Britain in the 1970s. 
X-rays are also used by a fluoroscope for examining baggage at airports, bomb squads, or for medical examinations where the Doctor requires real-time displays.
- ↑ Nobel Prize in Physics
- ↑ X-rays - School for Champions: Succeed in Physical Science
- ↑ http://www.orau.org/ptp/collection/shoefittingfluor/shoe.htm
- Medical diagnostics using X-rays - long, technical article written on high-school or college level