The Duane–Hunt limit describes the shortest possible wavelength of a photon emitted by an X-ray tube.
An X-ray tube accelerates electrons through a voltage and then rapidly decelerates them in a metal target. The highest-energy bremsstrahlung photon can receive, at most, the electron’s entire kinetic energy. Equating that energy with the photon energy gives the minimum wavelength: λmin = hc/eV.
William Duane and Franklin L. Hunt established this relationship experimentally in 1915. It is a direct demonstration that electromagnetic radiation is exchanged in discrete energy quanta. Increasing the tube voltage raises the maximum photon energy and shortens the minimum wavelength; changing the target material mainly changes the characteristic lines superimposed on the continuous spectrum.
The limit is not the wavelength of every emitted X-ray. Most photons have lower energies and longer wavelengths. It is the sharp short-wavelength cutoff of the continuous spectrum, making it useful for checking X-ray tube voltages.