College Physics (4th Edition)

$\lambda = 6.21\times 10^{-11}~m$
We can find the minimum wavelength: $q~\Delta V = \frac{hc}{\lambda}$ $\lambda = \frac{hc}{q~\Delta V}$ $\lambda = \frac{(6.626\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{(1.6\times 10^{-19}~C)(20.0\times 10^3~V)}$ $\lambda = 6.21\times 10^{-11}~m$