An X-ray beam emerging from an X-ray tube

The X-ray beam emerging from an X-ray tube consists of continuous X-rays (Bremsstrahlung) and characteristic X-rays.

The continuous X-ray spectrum has a sharp lower limit for the wavelength, known as the cut-off wavelength or minimum wavelength, given by \lambda_\min = \dfrac{hc}{eV}, where $V$ is the accelerating voltage across the tube.

There is no theoretical upper limit to the wavelength of the continuous X-rays produced, meaning the spectrum extends to longer wavelengths with decreasing intensity.

Therefore, the X-ray beam contains all wavelengths longer than a specific minimum wavelength.