A beam of monochromatic light is used to excite the electron in ${\mathrm{Li}}^{++}$ from the first orbit to the third orbit. The wavelength of monochromatic light is found to be $x\times {10}^{-10} \mathrm{m}$. The value of $x$ is _____ .

[Given $hc=1242 \mathrm{eV} \mathrm{nm}$]