Wavelength of yellow light = 580 nm = 580 × $10^{-9}$ m [Since 1 nm = $10^{-9}$ m] Frequency (u) = $\frac{c}{\lambda}$ , where c = velocity of light = 3.0 × 108 m $s^{-1}$, λ = wavelength of sodium lamp u = $\frac{3.0 \times 10^{8}}{580 \times 10^{-9}}$ = 5.17 × $10^{14} \mathrm{s}^{-1}$ Again, wavenumber $\left(\overset{-}{v}\right)$ = $\frac{1}{\lambda}$ = $\frac{1}{580 \times 10^{-9} \mathrm{m}}$ = 1.724 × $10^{6} \mathrm{m}^{-1}$