In an x-ray tube having a copper metal as a target, electrons with 20kev energy send to the target material to produce x rays.  Please explain in detail why X-rays having different wavelengths (between 0.6-3.0 Angstrom) are formed instead of a formation of X-rays with a single wavelength. In other words, why continuous x-ray spectrum composed of a broad range of wavelengths instead of a single wavelength, what is the physical explanation?