X-rays

X-rays were discovered by the German physicist W. C. Rontgen in 1895 while he was studying cathode rays (a stream of electrons emitted from the cathode inside a vacuum tube); they are accordingly also called Rontgen rays. X-rays are produced when a beam of cathode-ray electrons is suddenly stopped by striking an obstacle or target.

Because X-ray photons carry such high energy, X-rays have high penetrating power -- they can pass through several centimetres of solid matter, which is what allows them to be used to visualise the interior of materials that are opaque to ordinary visible light. Along with ultraviolet and gamma rays, X-rays are classed as ionizing radiation: they carry enough energy to strip electrons away from the atoms and molecules lying along their path, turning those atoms into ions. This ionizing ability is exactly what makes an overdose of X-rays harmful to human beings -- ionization can kill or damage living cells, or cause them to grow abnormally as cancers -- so exposure must always be kept within safe limits. X-rays have many practical applications precisely because of their high penetrating power: they are used to study the structure of crystals, to take medical X-ray p …