Duane–Hunt law

The maximum frequency νmax is given by[2] which corresponds to a minimum wavelength where h is the Planck constant, e is the charge of the electron, and c is the speed of light.

In an X-ray tube, electrons are accelerated in a vacuum by an electric field and shot into a piece of metal called the "target".

The continuous spectrum is due to bremsstrahlung, while the sharp peaks are characteristic X-rays associated with the atoms in the target.

The spectrum has a sharp cutoff at low wavelength (high frequency), which is due to the limited energy of the incoming electrons.

A photon with energy of 60 keV or less has a wavelength of 21 pm or more, so the X-ray spectrum has exactly that cutoff, as seen in the graph.

Spectrum of the X-rays emitted by an X-ray tube with a rhodium target, operated at 60 kV . The continuous curve is due to bremsstrahlung , and the spikes are characteristic K lines for rhodium. The Duane–Hunt law explains why the continuous curve goes to zero at 21 pm .