From the expression,

λ=c/v

we get,

v=c/λ

Where, ν = frequency of yellow light

c = velocity of light in vacuum = 3 × 10^{8} m/s

λ = wavelength of yellow light = 580 nm = 580 × 10^{–9} m

Substituting the values in expression (i):

v=3*10^{8}/580*10^{-9}=5.17*10^{14} s^{-1}

Thus, frequency of yellow light emitted from the sodium lamp = 5.17 × 1014 s^{–1}

Wave number of yellow light v=1/λ

1/580*10^{-9}=1.72*10^{6} m^{-1}