Wavelength of yellow light $=240$ nm
                                 = 240 x 10^-9 m
Wave number $\left(\overline{v}\right)=\frac{1}{\lambda }=\frac{1}{240\times {10}^{-9}\mathrm{m}}=4.16\times {10}^6{\mathrm{m}}^{-1}$