The cutoff wavelength of X-rays emitted by a Coolidge tube depends...

The cutoff wavelength of X-rays is the minimum wavelength of the continuous X-ray spectrum. It corresponds to the maximum kinetic energy of the striking electrons being completely converted into a single X-ray photon.

The maximum energy of an X-ray photon is given by E_\max = eV, where $V$ is the accelerating voltage.

Since E_\max = \dfrac{hc}{\lambda_\min}, we have \dfrac{hc}{\lambda_\min} = eV.

\lambda_\min = \dfrac{hc}{eV}

Thus, the cutoff wavelength depends only on the accelerating voltage applied across the Coolidge tube.