W. K. Roentgen discovered

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 10¹⁶ to 10²⁰ hertz (Hz).