Why do sodium-vapour lamps give off yellow light?

Sodium-vapour lamps emit a characteristic yellow light because of the specific way sodium atoms behave when electrically excited.

When an electric current passes through the lamp, it vaporizes the metallic sodium and raises its atoms into high-energy states. As these atoms decay back to their ground state, they release energy in the form of photons, which manifests as a nearly monochromatic yellow light. This light consists primarily of two specific wavelengths—5890 and 5896 angstroms—which correspond to the D2 and D1 Fraunhofer lines.123

While low-pressure sodium lamps produce this distinct yellow glow, high-pressure sodium lamps incorporate other elements, such as mercury, to produce a whiter light suitable for applications where a broader spectrum of reflected colours is required.1