If ${\mathrm{\lambda }}_{\mathrm{Cu}}$ is the wavelength of ${\mathrm{K}}_{\mathrm{\alpha }}$ X-ray line of copper (atomic number 29) and ${\mathrm{\lambda }}_{\mathrm{Mo}}$ is the wavelength of the ${\mathrm{K}}_{\mathrm{\alpha }}$ X-ray line of molybdenum (atomic number $42$), then the ratio $\frac{{\mathrm{\lambda }}_{\mathrm{Cu}}}{{\mathrm{\lambda }}_{\mathrm{Mo}}}$ is close to