Light is produced by gaseous-conduction methods, when proper energy transistions result from electron displacement within the atomic structure of the gas involved.
Applied voltage at the electrodes accelerates free electrons from their normal atomic positions. Radiations of a particular wavelength result as the displaced electons return to their normal position in the atomic structure; this wavelength depends on the base used and the degree of electron displacement.
Once the discharge begins, the enclosed artc becomes a light source with one electrode acting as a cathode and the other as an anode. The electrodes will exchange functions as the supply changes polarity. This principal is employed in high-intensity discharge lamps.
source; AE Handbook, 10.114, 142