Light from sodium lamp is passed through cold sodium vapours, the spectrum of transmitted light consists of

$$\begin{gathered} \text{ We know that }\frac{{\mathrm{\delta }}_{\mathrm{v}}-{\mathrm{\delta }}_{\mathrm{r}}}{{\mathrm{\delta }}_{\text{mean }}}=\mathrm{\omega } \\ \Rightarrow \text{ Angular dispersion }={\mathrm{\delta }}_{\mathrm{v}}-{\mathrm{\delta }}_{\mathrm{r}}=\mathrm{\theta }={\mathrm{\omega \delta }}_{\text{mean }} \end{gathered}$$

According to Kirchhoff's law, a substance in the unexcited state will absorb these wavelengths which it emit in de-excitation. Hence no spectral features.