Yellow light emitted from a sodium lamp has a wavelength ( λ) of 580 nm....

From the wording,
$\lambda =\frac{c}{v}$
we get,$v=\frac{c}{\lambda }$.............(i)
Where, ν = frequency of yellow light
c = velocity of light in vacuum $=3\times {10}^{8}m/s$
$\lambda$ = wavelength of yellow light $=580nm=580\times {10}^{-9}m$ Substituting the values in expression (i):
$v=\frac{3\times {10}^8}{580\times {10}^{-9}}=5.17\times {10}^{14}{s}^{-1}$
As a result, the sodium lamp's yellow light emission frequency $=5.17\times {10}^{14}{s}^{–1}$
Wave number of yellow light $$\begin{gathered} \overline{v}=\frac{1}{\lambda }, \\ =\frac{1}{580\times {10}^{-9}}=1.72\times {10}^6{m}^{-1} \end{gathered}$$