W. K. Roentgen discovered

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Wilhelm Roentgen, or W K Roentgen Professor of Physics in Wurzburg, Bavaria, discovered X-rays in 1895—accidentally—while testing whether cathode rays could pass through glass. X-ray, electromagnetic radiation of extremely short wavelength and high frequency, with wavelengths ranging from about 10 −8 to 10 −12 metre and corresponding frequencies from about 10 16 to 10 20 hertz (Hz).