X-rays are a form of high-energy electromagnetic radiation, with wavelengths shorter than those of ultraviolet rays and longer than those of gamma rays – roughly in the range of 0.01 to 10 nanometres. X-rays were discovered by German scientist Wilhelm Röntgen in 1895, who named them X-radiation to signify an unknown type of radiation. X-rays can penetrate many solid substances such as construction materials and living tissue, and X-ray radiography is widely used in medical diagnostics. This medical significance was noticed by Röntgen shortly after he discovered X-rays; this print, titled Hand mit Ringen (Hand with Rings), is a print of his first medical X-ray, taken of his wife Anna Bertha Ludwig's hand in December 1895.Print credit: Wilhelm Röntgen; restored by Yann Forget