In 1895,the German physicist roentgen(W).C.ROntgen)in the study of cathode ray tube gas discharge phenomenon,has two metal electrodes embedded in a(called a anode,called a cathode)sealed glass tube,on both ends of the electrode of tens of thousands of volts of high voltage,with the pump from the air inside the glass tube.In order to hide the light from the high voltage discharge(a kind of arc)leakage,cover the glass tube with a layer of black cardboard.When he carried out the experiment in the darkroom,he came across the bright fluorescence of a piece of pasteurized cardboard,two meters away from the glass tube.Further experiments,using cardboard,wood,clothing and a book of about two thousand pages,can't cover the fluorescence.Even more surprising was the sight of the hand bone on the cardboard when the hand went to pick up the fluorescent cardboard.

　　At the time,roentgen decided that it was a ray that people could not see but could penetrate.Unable to explain its principle and its nature,it borrows the"X",which represents the unknown in mathematics,as the"X"ray(or X-ray or X ray).This is where the X-ray findings and names come from.The name has been extended to this day.Later,the great discovery of roentgen was called the roentgen ray.

　　X-ray findings has very important significance in the history of mankind,it is the natural sciences and medicine opened up a new path,for roentgen won the first Nobel Prize in physics in 1901.

　　Science is always in constant development,the roentgen and repeated practice and research,scientists from all over the world gradually reveals the essence of the X-ray,confirmed that it is a kind of very short wavelength,energy a lot of electromagnetic waves.Its wavelength is shorter than the wavelength of visible light(about 0.From 001 to 100nm,the X-ray wavelengths used in medicine are about 0.001.~0.Between 1nm,its photon energy is tens of thousands to hundreds of thousands of times greater than that of visible light.Therefore,X-ray has its own characteristics besides the general properties of visible light.