All X-ray technologists are taught the history of x-rays. There is a chance you may see a question or two regarding this on you Limited Scope ARRT Exam, so we'll briefly cover it.
X-ray's were discovered in 1895 by German physicist Wilhelm Roentgen.
Roentgen was working with electricity through various types of vacuum tubes in order to study the properties of electrical discharge. During his studies, he noticed a part within one of his set ups "glowing". The part had been coated with a substance that caused it to fluoresce (glow) when struck by x-rays. This would mark the first time direct observation of x-ray interaction was seen. X-rays, like many human discoveries, were discovered somewhat by accident.
Roentgen soon realized this was a new kind of energy, some kind of "invisible light", and gave it the name X-light, which was later changed to X-radiation. The "X" stood for unknown. Although he discovered many of the properties x-rays have, including the ability to penetrate the human body, he was still unaware of the full impact of exactly what x-rays were. He certainly did not understand the dangers that can be associated with x-rays. Radiation would not be discovered until the following year. Today we recognize that x-rays are a form of radiation that can cause cancer, which is rather strange considering we also use it to fight cancer.
*This image is the first medical image ever taken, the hand of Wilhelm Roenten's wife.