What produces light when sodium reacts with halogen vapours?

Under certain reaction conditions, sodium and halogen vapours react to produce light through a process known as chemiluminescence. This phenomenon occurs when the chemical energy released during the reaction is emitted as light.1

It is important to distinguish this from other forms of light emission associated with the element: while chemiluminescence occurs during specific chemical reactions, the characteristic yellow glow often associated with sodium—such as that seen in sodium-vapour lamps—is typically produced through electrical excitation or thermal excitation, which causes sodium atoms to emit photons as they decay from high energy states back to their ground state.23