Explanation

(a) If all of the kinetic energy carried by an electron is converted into radiation, the energy of the X-rays photon would be given by
${E\ }_{\max} = {hv\ }_{\max} = eV$
Where $h$ is Planck’s constant, ${v\ }_{\max}\ $the largest frequency, $e$ charge of an electron and $V$ the applied voltage.
This maximum energy or minimum wavelength is called the Duane-Hunt limit.
$\therefore\ \ \ \ \ \ \ \ {hv\ }_{\max} = \frac{hc}{{\lambda\ }_{\min}} = eV$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ {\ \lambda\ }_{\min} = \frac{hc}{eV}$